buildman: Allow using older versions of genboardscfg.py

Older versions of this script don't support the -q flag. Since buildman
runs this script from when it starts, we may get the old version.

Fix this in two ways:

1. Use the version from the same tree as buildman is run from, if
available
2. Failing that, allow the -q flag to be missing

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