Rename inconsistently named testing-related variables. Also fix STM32CUBE not working from application code.

- 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

Clean up top-level build file and build options, fix build issues on MinGW

rtos: Thread: Make stack allocation failure runtime catchable ...

When a Thread object's stack memory is not provided, its `start()`
member function dynamically allocates its stack from the heap. If
allocation fails, there is no way to catch it because
* `std::nothrow` is missing after the `new` keyword. As Mbed OS
is built with `-fno-exceptions` (C++ exceptions disabled), failed
allocation results in an unrecoverable fault.
* The attempted `nullptr` check, which doesn't work anyway due to
the first point, is an assertion instead of error returning.
Assertions should be used as a development tool to ensure code
behaves correctly. But out-of-memory is a completely valid runtime
situation.

This commit adds the missing `std::nothrow`, and makes `Thread::start()`
return `osErrorNoMemory` if allocation fails so the caller can handle
it.

Note: A case when a thread should never fail due to lack of memory
is the main thread. But the main thread's stack is a pre-allocated
array in the static memory, passed to the `Thread()` constructor
during thread creation, so it's not impacted by this change.

Merge pull request #15045 from LDong-Arm/malloc_newlib_nano ...

Workaround for a bug in malloc() from newlib-nano 4.1.0 (in GCC 10.3)

Incorporate the review comments

Revert "TESTS: Reduce allocate memory size" ...

This reverts commit 6649e95.

In the malloc test, `ALLOC_ARRAY_SIZE` defines the *capacity* of array
that stores pointers to `malloc`'d buffers. It is *not* the number
of allocations.

In an ideal scenario, the test makes as many allocations as possible
until the heap runs out and malloc() returns NULL. So revert the
capacity of the array of pointers from 50 back to 100 so this array
is less likely to run out before the heap does.

Update rtos CMake to include greentea test dir

CMake: greentea: Add skip reason to rtos test

CMake: Refactor rtos greentea test CMake ...

Initially, every library greentea test has its project() creation in
their CMake. As running greentea using CTest move all greentea test
suite under one global project mbed-os and MBED_CONFIG_PATH set at
the root mbed os CMake under the condition BUILD_GREENTEA_TESTS
check so refactored rtos greentea CMake accordingly.

TESTS: Reduce allocate memory size

CMake: Add option to enable greentea tests ...

Add an option to enable the greentea tests independently from the unit
tests.

We can't just use the typical BUILD_TESTING option to enable greentea
tests. BUILD_TESTING enables unit tests and fetches googletest, which
are compiled for the host. Greentea tests are cross compiled and require
a toolchain file. For this reason we add a new option just to enable
greentea tests, preventing build failures triggered by the unit tests
and googletest.

Unit tests: Remove redundant CMake target mbed-headers-base ...

Individual libraries' `target_h` stub headers have now all been moved
from `mbed-headers-base` to `mbed-headers-<library>`.

Note: Even though headers previously in `target_h` are technically
stubs/fakes too, they are used by not only unit tests but also regular
libraries when compiled for unit tests, because no target-specific HAL
implementation exists in this case. In order for regular library
sources to pick up `target_h` headers, those headers must

* have the same names as regular headers
* appear first in include paths

This is why those headers are part of `mbed-headers-<library>` and not
`mbed-stubs-<library>`. Before this refactoring, `mbed-headers-base`
was the first in unit tests' include paths.

Unit tests: Fix style in rtos Thread.h stubs

Unit tests: Move rtos target_h stubs

rtos: fix delay with 0 ...

Fixes error osErrorParameter that triggers our assert.
RTX delay ignored 0 previously, it now returns osErrorParameter instead.

CMake: Remove mbed-stubs-rtos-headers library ...

- Previous changes moved all rtos stubs headers into mbed-stubs-rtos-headers
lib, but the decision to keep all stubs headers under the respective
component stubs library so moved all stubs rtos headers under
mbed-stubs-rtos and updated it depend component CMake
- Remove unnecessary add_definition call for UNITTEST as any of the stubs library
added from UNITTEST/CMakeLists.txt is not required this macro

rtos: Remove UNITTEST macro from rtos ...

Production code should not contain any test-specific checks. Rather
than checking `UNITTEST`, unit tests can directly set
`MBED_CONF_RTOS_PRESENT=1` to make RTOS available for testing.

Note: The cellular ATHandler test also has `MBED_CONF_RTOS_PRESENT=1`
added because `ATHandler.cpp` contains a check of this variable.

Merge pull request #14718 from Meano/develop ...

Feature: Make changes for Cortex-A5 support

Merge pull request #14819 from ARMmbed/move_rtos_stubs ...

Move rtos stubs to the rtos directory

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: unit-tests: Move mbed-headers-rtos to the rtos directory ...

Move the header-only mbed-headers-rtos library the unit test stubs
depend on into the rtos component directory. This makes the rtos
stubs more self-contained and improves the composition of the library.

CMake: Move rtos stub headers to the rtos/tests/UNITTESTS/double directory ...

Move rtos stubs headers to the mbed-os/rtos/tests/UNITTESTS/double
directory and update CMake to create a mbed-stubs-rtos-headers
library for rtos stub headers

CMake: Move rtos stubs sources to the mbed-os/rtos directory ...

Move the rtos stub library into the rtos component directory.
so we can avoid duplicating the mbed-os source tree in a central
UNITTESTS folder.

Make changes for Cortex-A5 support

Better document MemoryPool behaviour ...

Make explicit that `MemoryPool` is a raw allocator, and users need to
take care of object construction and destruction.

CMake: rename greentea test macro

CMake: Add CMake support for rtos greentea test

Fix ThisThread warnings

Merge pull request #13085 from pea-pod/remove-deprecated-mbed-assert ...

Change MBED_STATIC_ASSERTs version for built-in