buildman: Allow ignoring warnings in the return code

Sometimes we don't want buildman to return failure if it seems warnings.
Add a -W option to support this. If buildman detects warnings (and no
errors) it will return an exit code of 0 (success).

Note that the definition of 'warnings' includes the migration warnings
produced by U-Boot, such as:

    ===================== WARNING ======================
    This board does not use CONFIG_DM_MMC. Please update
    ...
    ====================================================

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>