buildman: Implement an option to exclude boards from the build

Some boards are known to be broken and it is convenient to be able to
exclude them from the build.

Add an --exclude option to specific boards to exclude. This uses the
same matching rules as the normal 'include' arguments, and is a comma-
separated list of regular expressions.

Suggested-by: York Sun <yorksun@freescale.com>
Signed-off-by: Simon Glass <sjg@chromium.org>