tools/genboardscfg.py: check if the boards.cfg is up to date

It looks silly to regenerate the boards.cfg even when it is
already up to date.

The tool should exit with doing nothing if the boards.cfg is newer
than any of defconfig, Kconfig and MAINTAINERS files.

Specify -f (--force) option to get the boards.cfg regenerated
regardless its time stamp.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>