Addresses various compatibility/usability issues observed during testing

- Correctly set/export the CMAKE_EXE_EXPORTS_C_FLAG
- Incorporate the system specs directly into the COMPILE_OBJECT templates
  so these don't need to be specified by the toolchain or CMAKE_C_FLAGS
- Similarly include the always-needed linker flags flags for entry point
  and relocation address into the linker command template

Do not use aux_source_directory to assemble a list of source files.
Instead, put a proper CMakeLists.txt file in each implementation
and build the implementation separately from the shared/common parts.

In addition to avoiding the aux_source_directory this allows PSP-specific
compile definitions to be set on a per-implementation basis because
it is defined separately.

Instead of redefining the basic entry point symbols, use the
OSAL BSP and its abstractions.  The CFE PSP just becomes an
extension to the OSAL BSP, and only needs to define the
OS_Application_Startup symbol like any other OSAL application.

Fix #121, Unify CFE PSP with OSAL BSP

Fix #145, Update RTEMS CMake module

Fix #135, Add separate CMakeLists.txt for each implementation