Improve PSP config override concept

[skliper]

Is your feature request related to a problem? Please describe.
Historically PSP's have been treated as clone-and-own, but to be a bit friendlier for reuse it would help to support a more general configuration override concept that wouldn't break unit testing.

Example issue - if a user overrides cfe_psp_config.h for a specific target, it ends up getting used by unit test for the other PSPs that rely on unique/conflicting cfe_psp_config.h elements. This can be avoided by conditionally NOT pulling in a cfe_psp_config.h override for native or when ENABLE_UNIT_TESTS=1 unless trying to unit test the system it applies to but this really isn't pretty if it's a multi-build and you really do want to coverage test using overridden configs. Basically a user had an override to the linux PSP config, but unit tests build for vxworks coverage and they tried to use the linux PSP coverage override.

Describe the solution you'd like
Allow for easily disabling PSP unit tests for the non-target PSPs. Likely a good thing for OSAL also. Really users only need to test what they are using, just the open source bundle needs to test them all.

Also would help to transition to the OSAL pattern for setting cmake variables, and use those to set the defines with default backups. The built configs could then easily override just what they want, and the common-across-psp settings would be portable. Maybe the config headers should have unique names associated with the implementation vs all being called cfe_psp_config.h since really they aren't portable. Or separate the unique and portable settings.

Describe alternatives you've considered
None

Additional context
None

Requester Info
Jacob Hageman - NASA/GSFC, from conversation with @excaliburtb