cFE 6.6: Add a CMakeLists file for building the ECI source

[BaldBeacon]

There is a duplication of effort when compiling the ECI source files with a project. So far both the simple app and sgp4 prop examples both require some steps to include the ECI source to compile the app. It would be nice, for build simplicity, to include a CMakeLists in the ECI repository that given a list of source files will compile external code along side ECI. This system could be used to remove the need for each app to specify the directories of ECI's source which was a source of duplicate code.