Fix #790, allow directories and symlinks in FILELIST.

[jphickey]

Describe the contribution
Checks a name-based subdirectory under ${MISSION_DEFS} for files listed in the FILELIST for that target. If file is a symlink, the link should be followed so the correct content is installed, not a symlink.

Fixes #790

Testing performed
Created a symlink in a target name subdirectory (see additional context) and confirm that the correct file content is installed.

Expected behavior changes
Items in FILELIST can be in a target name directory, and may be symlinks.

System(s) tested on
Ubuntu 20.04

Additional context
This also adds another "NOTE" message to indicate exactly what file is being used, after all lookup/de-mapping logic is done, e.g. I made a symlink to test this out:

$ cd sample_defs
$ mv cpu1_cfe_es_startup.scr test_startup.scr
$ mkdir cpu1
$ cd cpu1
$ ln -s ../test_startup.scr cfe_es_startup.scr
$ cd ../..
$ make prep
<<snipped>>
NOTE: Selected /home/joe/code/cfecfs/github/sample_defs/test_startup.scr as source for cfe_es_startup.scr on cpu1

This is similar to all the other informational messages for table sources and app sources indicating where they were found.

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

[astrogeco]

CCB 2021-02-17 APPROVED

• Supports having different versions of files for different CPUs by using directories in MISSION_DEFS