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>