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.
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>