buildman: run genboardscfg.py all the time

This commit makes sure boards.cfg is up to date before starting
the build tests.  tools/genboardscfg.py exits immediately printing
"boards.cfg is up to date. Nothing to do." when boards.cfg is
already new.

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