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>
diff --git a/tools/buildman/cmdline.py b/tools/buildman/cmdline.py
index 74b4100..f387aeb 100644
--- a/tools/buildman/cmdline.py
+++ b/tools/buildman/cmdline.py
@@ -108,6 +108,8 @@
           default=False, help='Run make with V=1, logging all output')
     parser.add_option('-w', '--work-in-output', action='store_true',
           default=False, help='Use the output directory as the work directory')
+    parser.add_option('-W', '--ignore-warnings', action='store_true',
+          default=False, help='Return success even if there are warnings')
     parser.add_option('-x', '--exclude', dest='exclude',
           type='string', action='append',
           help='Specify a list of boards to exclude, separated by comma')