buildman: Add a functional test

Buildman currently lacks testing in many areas, including its use of git,
make and many command-line flags.

Add a functional test which covers some of these areas. So far it does
a fake 'build' of all boards for the current source tree.

This version reads the real ~/.buildman and boards.cfg files. Future work
will improve this.

Signed-off-by: Simon Glass <sjg@chromium.org>