Add a -b option which scans all the defconfigs and builds a database of
all the CONFIG options used by each. This is useful for querying later.

At present this only works with the separate -b option, which does not
move any configs. It would be possible to adjust the script to build the
database automatically when moving configs, but this might not be useful
as the database does not change that often.

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