Make PSP more modular

[jphickey]

Is your feature request related to a problem? Please describe.
It is currently not easy to override items in the "shared" directory even for platforms which it does not apply.

For example, in the "cfe_psp_ram.c" and "cfe_psp_port.c" files this contains code that directly writes to physical memory addresses, which may work on mcp750, but will likely segfault on an pc-linux system.

The code should be better structured as modules so the "correct" implementation can be used for each system without breaking the other system.

Describe the solution you'd like
Most of the code in shared should be modularized so it can be selectable per-platform.
Each platform (mcp750, pc-linux, pc-rtems, plus whatever others users might have) then becomes just a collection of modules.

Additional context
There really shouldn't be much in the shared directory (the existence of such a directory implies its not platform-specific, but yet its in the PSP).

Requester Info
Joseph Hickey, Vantage Systems, Inc.

[jphickey]

I forgot about old issue #10 before submitting this one, which is kind of a different take on the same thing.

The fundamental issue is we have code in the shared subdirectory which is definitely not generic/cross-platform, yet built and linked unconditionally into all platforms, with no easy/documented way to customize it and/or avoid it.