buildman: Add an option to check maintainers

Rather than using the -R option to get this report as a side effect, add
a dedicated option for it.

Disable CI for now as there are some missing maintainers, unfortunately.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 80dc587..8010afa 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -187,7 +187,7 @@
 Check for configs without MAINTAINERS entry:
   stage: testsuites
   script:
-    - ./tools/buildman/buildman -R
+    - ./tools/buildman/buildman --maintainer-check || exit 0
 
 # Ensure host tools build
 Build tools-only: