Use separate source files and CMake-based source selection
to configure OSAL features, rather than using the C preprocessor.

All implementation units now provide a separate header
file for each functional group/subsystem.

This commit covers the OSAL main FSW code and particularly the
"portable" implementation blocks to follow this model.  Includes
minor updates to UT stubs due to header file changes.

Use separate source files and CMake-based source selection
to configure OSAL features, rather than using the C preprocessor.

This commit covers the OSAL coverage test.  This had been using
inclusion of a C file to allow access to static functions/variables.
This updates all UT code to correspond with the FSW code changes.
All "static" helper functions become non-static so unit test can
invoke directly.  Prototype is in a private header, so
the functions are still not exposed in the API.

Permissive mode is required for testing on POSIX when
running as a normal (non-root) user.

Fix #285, Refactor OSAL to avoid inclusion of C files

The "include()" cmake function only read the first file when passed a list.
Adding a foreach loop supports allowing a list of files which is useful
for adding and override/addendum file to a base config.

Definitions within osconfig.h are referenced by doxygen
in other source code files, so this should also include
doxygen-formatted descriptions of each macro defined by
this template file.