Fix #2289, implement common search routine for config files

[jphickey]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
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Describe the contribution
Adds a 'cfe_locate_implementation_file()' function that provides a consistent means to find a file within the ${MISSION_DEFS} subdirectory.

Similar locate/search logic was necessary in a few places, for generate_config_includefile as well as add_cfe_tables, but these were not consistent in the paths or ordering used. Using this function should make them consistent.

Fixes #2289

Testing performed
Confirm that the new file matching routine finds the same file as the old logic did

Expected behavior changes
More flexibility with generate_config_includefile()

System(s) tested on
Debian

Additional context
Currently this PR only employs the new logic for generate_config_includefile(). Separate PRs will switch add_cfe_tables() and any other logic used to search for other files.

Contributor Info - All information REQUIRED for consideration of pull request
Joseph Hickey, Vantage Systems, Inc.

[jphickey]

Follow-on work (assuming this can get merged in next cycle) would be to fix up these other search loops to use the same function:

cFE/cmake/arch_build.cmake

Line 703 in 7af467e

foreach(INSTFILE ${${TGTNAME}_FILELIST})

cFE/cmake/target/CMakeLists.txt

Line 217 in 7af467e

if(EXISTS ${MISSION_DEFS}/${TGTNAME}_${EMBFILE})

cFE/cmake/arch_build.cmake

Line 187 in 7af467e

if (EXISTS "${MISSION_DEFS}/tables/${TGT}_${TBLWE}.c")

The last one (for add_cfe_tables) will be fixed as part of what I'm working on for #2284 ...