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

Convert emac, cellular, and wifi targets, and enable one UDP test

Convert nanostack and lwipstack to STATIC libraries

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

cmake : enable wifi in netsocket

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.

CMake: Only build unit tests if Mbed OS is the current project ...

Typically when adding a unit test directory to a CMake project a check
will be used to ensure the subdirectory is added only if the following
are true:

* The BUILD_TESTING option is set to ON.
* The current CMake project is the top-level project.

The reason being, if a downstream project includes our project they
generally don't want to build our unit tests.

In mbed-os, we do correctly specify the above condition before adding
the central UNITTEST subdirectory, which fetches googletest and adds the
"stub" libraries the unit tests depend on. However, we only check if
CMAKE_CROSSCOMPILING is OFF (or undefined) before actually adding the
unit tests. This mismatched logic would lead to unexpected build
failures in various scenarios. One likely case could be: a downstream
project including mbed-os happens to set CMAKE_CROSSCOMPILING to
OFF/undefined for any reason (possibly to build its own unit tests).
mbed-os would go ahead and attempt to build its tests without fetching
googletest or adding the required stub targets.

To fix the issue replace the check for CMAKE_CROSSCOMPILING in the unit
tests with the same BUILD_TESTING idiom we use for adding the central
UNITTESTS subdirectory.

CMake: Add add_subdirectory of unittests ...

- add every libraries unittest directory into respective CMake
  which allows to include unittest source into build based on
  MBED_BUILD_UNITTESTS flag

CMake: Add more information to TODO comment

CMake: Rename CMake targets ...

* mbed-os renamed mbed-core
* mbed-os-<COMPONENT> renamed mbed-<COMPONENT>

CMake: Restore rtos api and netsocket presence macro names ...

The old tools rely on the old names to build

cmake: fix present inclusion ...

We use only _PRESENT macros for components if they are enabled. Use the same in
the mbed.h file or anywhere else.

CMake: Componentize Mbed OS into multiple CMake targets (#13732) ...

Aside from the core mbed-os CMake target, a number of targets have been created so they can optionally be included by application executables that require them using `target_link_libraries()`.

Co-authored-by: Martin Kojtal <martin.kojtal@arm.com>
Co-authored-by: Rajkumar Kanagaraj <rajkumar.kanagaraj@arm.com>

CMake: Fix build due to system_storage and netsocket dir move in master