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

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: Create mbed-headers-kvstore ...

Create a CMake target mbed-headers-kvstore to separate KVStore headers
from the generic mbed-headers-storage. Update tests to use it.

CMake: Add Blockdevice dependency to KVstore library ...

As KVstore relies on BlockDevice add the dependency
in Mbed OS so applications do not have to specify it

CMake: streamline kvstore support ...

Link KVStore dependency in Mbed OS so application projects do not have to.

KVStore: drop './' from CMake paths

KVStore: componentize CMake definitions into sub-components

CMake: Rename CMake targets ...

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

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 failure due to kvstore and cellular dirs changes

CMake: Add support for storage dir