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

BUG=None
TEST='factory install w/o firmware or extra rootfs: no exception on server or client.'

Review URL: http://codereview.chromium.org/2817038
diff --git a/autoupdate.py b/autoupdate.py
index 1ded29e..2db4b08 100644
--- a/autoupdate.py
+++ b/autoupdate.py
@@ -271,9 +271,12 @@
     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')