patman: set the default config_fname argument value to None

This better matches Python conventions, allowing to easily test
whether the optional argument is provided.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@savoirfairelinux.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/__main__.py b/tools/patman/__main__.py
index 8e32655..3748930 100755
--- a/tools/patman/__main__.py
+++ b/tools/patman/__main__.py
@@ -117,7 +117,7 @@
 argv = sys.argv[1:]
 args, rest = parser.parse_known_args(argv)
 if hasattr(args, 'project'):
-    settings.Setup(parser, args.project, '')
+    settings.Setup(parser, args.project)
     args, rest = parser.parse_known_args(argv)
 
 # If we have a command, it is safe to parse all arguments