buildman: Return an error if there are maintainer warnings

Detect warnings about missing maintain info and return result code 2 in
that case.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/control.py b/tools/buildman/control.py
index 79ce2f6..0c75466 100644
--- a/tools/buildman/control.py
+++ b/tools/buildman/control.py
@@ -188,12 +188,12 @@
         board_file = os.path.join(options.output_dir, 'boards.cfg')
 
         brds = boards.Boards()
-        brds.ensure_board_list(board_file,
-                               options.threads or multiprocessing.cpu_count(),
-                               force=options.regen_board_list,
-                               quiet=not options.verbose)
+        ok = brds.ensure_board_list(board_file,
+                                    options.threads or multiprocessing.cpu_count(),
+                                    force=options.regen_board_list,
+                                    quiet=not options.verbose)
         if options.regen_board_list:
-            return 0
+            return 0 if ok else 2
         brds.read_boards(board_file)
 
     exclude = []