Explicitly reply no update for factory clients w/o update.

autoupdate.py

Index: autoupdate.py
diff --git a/autoupdate.py b/autoupdate.py
index 8f9f5add803a449f5a14dc6b2aac0fcff71e55df..3e390b683cb71b6f6e1f69baec07455caf6ae635 100644
--- a/autoupdate.py
+++ b/autoupdate.py
@@ -262,9 +262,12 @@ class Autoupdate(BuildObject):
     for stanza in self.factory_config:
       if board_id not in stanza['qual_ids']:
         continue
+      if kind + '_image' not in stanza:
+        break
       return (stanza[kind + '_image'],
               stanza[kind + '_checksum'],
               stanza[kind + '_size'])
+    return (None, None, None)
 
   def HandleUpdatePing(self, data, label=None):
     web.debug('handle update ping')