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/control.py b/tools/buildman/control.py
index 5d80400..ded4360 100644
--- a/tools/buildman/control.py
+++ b/tools/buildman/control.py
@@ -386,6 +386,6 @@
                                 options.keep_outputs, options.verbose)
             if fail:
                 return 128
-            elif warned:
+            elif warned and not options.ignore_warnings:
                 return 129
     return 0