buildman: Ask genboardscfg to be quiet

Now that this tool has a 'quiet' flag, use it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/control.py b/tools/buildman/control.py
index c55a65d..3b41d7b 100644
--- a/tools/buildman/control.py
+++ b/tools/buildman/control.py
@@ -205,7 +205,7 @@
             os.makedirs(options.output_dir)
         board_file = os.path.join(options.output_dir, 'boards.cfg')
         genboardscfg = os.path.join(options.git, 'tools/genboardscfg.py')
-        status = subprocess.call([genboardscfg, '-o', board_file])
+        status = subprocess.call([genboardscfg, '-q', '-o', board_file])
         if status != 0:
             sys.exit("Failed to generate boards.cfg")