Support overriding msgids from a single project file

[skliper]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I reviewed the README file to see if the feature is in the major future work.
• I performed a cursory search to see if the feature request is relevant, not redundant, nor in conflict with other tickets.

Is your feature request related to a problem? Please describe.
LC doesn't allow overriding msgids from a single file:

LC/arch_build.cmake

Lines 21 to 27 in ca47670

	foreach(LC_CFGFILE ${LC_PLATFORM_CONFIG_FILE_LIST})
	set(DEFAULT_SOURCE "${CMAKE_CURRENT_LIST_DIR}/config/default_${LC_CFGFILE}")
	generate_config_includefile(
	FILE_NAME "${LC_CFGFILE}"
	FALLBACK_FILE "${DEFAULT_SOURCE}"
	)
	endforeach()

Describe the solution you'd like
Interestingly enough, it does support custom overrides for some of the other default files:

LC/mission_build.cmake

Lines 50 to 62 in ca47670

	foreach(LC_CFGFILE ${LC_MISSION_CONFIG_FILE_LIST})
	get_filename_component(CFGKEY "${LC_CFGFILE}" NAME_WE)
	if (DEFINED LC_CFGFILE_SRC_${CFGKEY})
	set(DEFAULT_SOURCE "${LC_CFGFILE_SRC_${CFGKEY}}")
	else()
	set(DEFAULT_SOURCE "${CMAKE_CURRENT_LIST_DIR}/config/default_${LC_CFGFILE}")
	endif()
	generate_config_includefile(
	FILE_NAME "${LC_CFGFILE}"
	FALLBACK_FILE "${DEFAULT_SOURCE}"
	)
	endforeach()

Not clear why this pattern wasn't followed for msgids, but it should be supported for all files. Until/unless there's a more generic way to override all msgid default files w/ a single file (should follow this pattern for performance id's also), related to nasa/cFE#2405.

Describe alternatives you've considered
It's possible to override w/ a file in target_defs that's a symbolic link, but this additional layer of abstraction/clutter in target_defs is undesirable.

Additional context
None

Requester Info
Jacob Hageman - NASA/GSFC