buildman: Ensure config_fname is inited

Init this variable at the top level since it is a global.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/bsettings.py b/tools/buildman/bsettings.py
index dcc200e..9b93b7a 100644
--- a/tools/buildman/bsettings.py
+++ b/tools/buildman/bsettings.py
@@ -5,6 +5,7 @@
 import os
 import io
 
+config_fname = None
 
 def Setup(fname=''):
     """Set up the buildman settings module by reading config files