GHS: Update selection of primaryTarget in MULTI project file
Changes to ``-A`` handling:
* Don't force CMAKE_GENERATOR_PLATFORM into cache when using default
value (breaks using CMake presets).
* Don't print message when using default value (breaks CMake tests).
Changes to ``GHS_PRIMARY_TARGET`` handling:
* Add as a cache variable so its known to GUI
* Don't always include``GHS_TARGET_PLATFORM``, it's only needed if
``GHS_PRIMARY_TARGET`` wasn't set by the user.
* Set ``GHS_PRIMARY_TARGET`` during platform selection instead of when
a language is enabled. By performing this sooner
``GHS_TARGET_PLATFORM`` is not always required to be set into cache.
3 files changed
