…sal-Users-Guide-From-ReadMe

Fix nasa#413, Add reference to osal user's guide from ReadMe

Integration Candidate: 2020-04-15

Use separate source files and CMake-based source selection
based on feature configuration, rather than using the C
preprocessor for including/excluding different OSAL function
groups.

Refactor all implementation units to provide a separate header
file for each functional group/subsystem.  Remove "static"
declaration on internal helper functions so they can be invoked
from unit test.