Use target_include_directories and target_compile_definitions to
propagate the interface details for PSP

Add a PSP implementation of the version API discussed in nasa/cFS#200

…256-version-api', 'jphickey/fix-254-psp-interface' and 'zanzaben/fix246_fix_comment_style' into ic-20210305

Adds a new target for PSP modules and use this to propagate the compile
definitions and include directories for all PSP code.

Fix nasa#261, add psp_module_api interface target

Certain operations in the recent baseline were dependent on a
recent version of CMake.  In particular object libraries cannot
be used in the intended way in the older versions.  This adds
workarounds for those cases.

Tested against CMake versions 3.5.1, 3.10.2, and 3.16.3.

Integration candidate: 2021-03-12

Update Readme, fix URL for IC-2021-03-12 entry in changelog

Use the PSP module feature to allow selection/override of EEPROM accessors.
The current code is converted to an "eeprom_direct" module which is included
only on platforms where it is valid (e.g. mcp750, no mmu).