Don't use whole-archive linking (#71) ...

* First attempt at converting to OBJECT libraries.  Not working due to more object library limitations

* Only compile the main libraries as OBJECT, because that's all that CMake can be made to do right now.

* Fix availability of target-specific flags

* Missed some usages

* Fix some STM32 ethernet stuff that was not linking correctly

* More fixes for STM32 ethernet and weak symbols

* Fix prebuilt object file linking for MUSCA_S1

* Fix STM32 eth init license headers

Change linker script handling logic to use only one linker script target

Fix Mbed OS target's public dependency on the MCU target

Fix PSA target, fix create_distro to not recurse into static libraries

Only support GCC ARM compiler, but reap the benefits of static libraries instead of objects

Convert the rest of the hal tests, convert mbedtls to an OBJECT library

Fix PSA and MUSCA build

- 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