buildman: Improve the config comparison feature

buildman: Improve the config comparison feature

At present buildman can compare configurations between commits but the
feature is less useful than it could be. There is no summary by architecture
and changes are not reported on a per-board basis.

Correct these deficiencies so that it is possible to see exactly what is
changing for any number of boards.

Note that 'buildman -b <branch> -C' is recommended for any build where you
will be comparing configuration. Without -C the correct configuration will
not be reported since changes will often not be picked up.

Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>