buildman: Set the return code to indicate build result

When buildman finds errors/warnings when building, set the return code to
indicate this.

Suggested-by: York Sun <yorksun@freescale.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/buildman.py b/tools/buildman/buildman.py
index e18859b..fbd3125 100755
--- a/tools/buildman/buildman.py
+++ b/tools/buildman/buildman.py
@@ -136,4 +136,5 @@
 
 # Build selected commits for selected boards
 else:
-    control.DoBuildman(options, args)
+    ret_code = control.DoBuildman(options, args)
+    sys.exit(ret_code)