These files are used in addition to the top-level prj.conf (or other
future top-level config files), NOT instead of it.

The build system finds board files in this directory based on the
BOARD setting. There's no need to modify CONF_FILE to include these
configurations in the build.

I.e. to build with prj.conf, just run:

     make BOARD=your-board

from the top-level. If your board needs extra configuration, the build
system will also merge in your-board.conf from this directory.