- Split mbed-core and mbed-rtos into -sources and -flags targets ...

- Fix some logic errors in top level cmakelists related to building greentea tests
- Fix mbed_create_distro() for OBJECT libraries

CMake: tests: Support skipping a test with a reason ...

Add a new argument `TEST_SKIPPED` to `mbed_greentea_add_test()` to
indicate a test is skipped and give a reason (e.g. the Mbed target
and/or configuration does not provide what the test requires).

The skip reason of a test is printed when running tests with `ctest`,
and the test is marked as "Skipped" in the test report.

cmake: Improve indentation of mbed_greentea_add_test doc

CMake: turn mbed_greentea_add_test() into a function ...

Unlike a macro, a function is more self-contained and can return
early if necessary.

CMake: greentea: Use more specific name for baremetal option ...

Make it obvious the option is used for greentea tests.

CMake: greentea: Port mbed-drivers-ticker to CTest ...

Call add_test in mbed_greentea_add_test and specify mbedhtrun as the
command to run.

An MBED_HTRUN_ARGUMENTS variable has been added, which is a semicolon
separated list of arguments to forward to htrun. The user is required to
pass in any arguments mbedhtrun needs to communicate with the device
under test at CMake configuration time.

We automate passing some of the htrun arguments in CMake using data
provided by mbed_config.cmake or gathered from the CMake environment.
The following arguments are passed to htrun from CMake:

* -f/--image-path
* -e/--enum-host-tests
* -m/--micro
* -R/--reset-timeout

--baud-rate is also passed if the MBED_CONF_PLATFORM_STDIO_BAUD_RATE
config parameter is defined.

Temporary checks have been added to mbed_greentea_add_test to keep the
old flow working until we port all of the greentea tests to CTest. These
checks should be removed after we make all greentea tests runnable by
CTest.

CMake: greentea: Fix io issue with greentea-client ...

The stdio implementation of the standalone greentea-client wasn't
streaming "KiVi protocol" messages through the serial IO. Separate out
the existing implementation for greentea-client that uses
mbed_retarget.h's `read` and `write` functions so greentea tests
continue to work with the standalone client.

The IO fixture is shared between the "legacy" embedded greentea-client
and the newer standalone client.

cmake: Use namespaced greentea client library ...

When using an exported or installed version of greentea, it's necessary
to use the greentea namespace.

Use standalone greentea-client ...

For CMake or Mbed CLI 2, use the standalone greentea-client.

CMake: Replace MBED_TEST_LINK_LIBRARIES with MBED_TEST_BAREMETAL ...

Currently we have `MBED_TEST_LINK_LIBRARIES` for specifying
* Whether to link `mbed-os` or `mbed-baremetal`
* Any additional libraries we want tests to link

It's not fit for purpose anymore, because
* No flavor of Mbed OS is selected by default, but we should've
really defaulted to `mbed-os`, the full RTOS version. Build doesn't
work unless `-DMBED_TEST_LINK_LIBRARIES=<...>` is passed, which
is redundant.
* A test should never need additional libraries passed via command
line - its `CMakeLists.txt` should specify what it links.

This commit replaces `MBED_TEST_LINK_LIBRARIES` with a new option
`MBED_TEST_BAREMETAL` to build a test with either RTOS (default)
or without it (by passing `-DMBED_TEST_BAREMETAL=ON`).

CMake: greentea: Remove dependency on global MBED_PATH ...

The mbed_greentea_add_test macro required a greentea test to set an
MBED_PATH variable to the path of the mbed-os root directory, which it
attempts to add as a 'subdirectory' of the test project. We can instead
use CMake's built in CMAKE_CURRENT_LIST_DIR variable to deduce the path
to mbed-os relative to the current list file directory, removing the
need for greentea tests to set MBED_PATH.

CMake: greentea: Remove call to deprecated function ...

mbed_greentea::mbed_greentea_add_test was calling the deprecated
`mbed_configure_app_target` function, which is currently a no-op.

CMake: mbed_greentea: Remove trailing whitespace

CMake: Require TEST_SOURCES in greentea CMake file ...

Assumption that greentea test file is always named main.cpp is
incorrect. Updated mbed_greentea_add_test() macro to make TEST_SOURCES
parameter compulsory, which is used to specify greentea test
file(s). This allows tests to use C, or have a different name.
Therefore also updated all pre-existing greentea test CMake files to
explicity add main.cpp to TEST_SOURCES.

CMake: Remove alias for TEST_NAME argument ...

The mbed_greentea_add_test macro previously set a variable in order to
use the un-prefixed TEST_NAME to refer to the argument in the macro
body. Whilst pair-programming with LDong, this was identified and
determined to be unecessary (maybe it was a failed attempt to fix
something, that was never reversed?) and so it has been removed.

Cmake: Add backward compatibilty support for MBED_TEST_MODE ...

Fixes #14494

MBED_TEST_MODE is required for backward compatibilty with CLI1. This
adds a test to ensure that the macro is created when using CLI2 for
testing. This also adds a test in `.travis.yml` that checks whether
CMake defines the macro when BUILD_TESTING is on.

CLI1 Reference: https://os.mbed.com/docs/mbed-os/v6.9/debug-test/greentea-for-testing-applications.html

Also, explicitly create and set the macro BUILD_TESTING to allow for
MBED_TEST_MODE to be defined by CMake. MBED_TEST_MODE is required for
backward compatibilty with CLI1.  BUILD_TESTING is used to determine
whether to define MBED_TEST_MODE.  Normally, this would be automatically
done by CTest (which we intend to add for test automation) but this
hasn't yet been added to our Greentea tests.

CMake tools: use CMAKE_CURRENT_LIST_DIR instead of MBED_PATH ...

Excluding greentea tests, they will be fixed separately (more work needed).

CMake: Fix Greentea tests ...

* Specify new Greentea tests build steps in README
* Correct path to generated CMake configuration module

CMake: Greentea test CMakeLists.txt refactoring ...

- Added the new CMake `MBED_TEST_LINK_LIBRARIES` command-line argument support to receive
  all dependent libraries for the particular greentea test suite.
For example:
- To select mbed-os library, use cmake with -DMBED_TEST_LINK_LIBRARIES=mbed-os
- To select baremetal library, use cmake with -DMBED_TEST_LINK_LIBRARIES=mbed-baremetal
- To select baremetal with external error logging library to the test with -D "MBED_TEST_LINK_LIBRARIES=mbed-baremetal ext-errorlogging"

cmake: Refactor mbed_set_mbed_target_linker_script ...

Move the mbed_set_mbed_target_linker_script function into its own file
and make it set the linkerscript on the target passed in, rather than
setting the linkerscript on the top level mbed-core target. We use
CMake's automatic forwarding of usage requirements to forward the linker
script to the top level target.

This commit also adds a condition to check whether the MBED_TARGET is a
CMake buildsystem target. If it is we will just link to it in the
idiomatic CMake way, if it's not then we use the 'old' mechanism of
setting the linkerscript for the mbed-core target. This check will be
useful when we refactor MBED_TARGETS to be CMake buildsystem targets as
it will allow us to do the refactoring incrementally.

CMake: rename greentea test macro

Apply suggestions from code review ...

Co-authored-by: Hugues Kamba <41612201+hugueskamba@users.noreply.github.com>

Incorporating the review comments

Create Greentea CMake Macro