buildman: Allow comparison of build configuration

It is useful to be able to see CONFIG changes made by commits. Add this
feature to buildman using the -K flag so that all CONFIG changes are
reported.

The CONFIG options exist in a number of files. Each is reported
individually as well as a summary that covers all files. The output
shows three parts: green for additions, red for removals and yellow for
changes.

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