Enable uprevving on preflight bot for this branch.

BUG=chrome-os-partner:5577
TEST=Run cbuildbot_config_unittest.py

Change-Id: I7018ef0cbedcdb11717d8e38d05ebef7424a0064
Reviewed-on: http://gerrit.chromium.org/gerrit/7116
Reviewed-by: Dave Parker <dparker@chromium.org>
Tested-by: David James <davidjames@chromium.org>
diff --git a/buildbot/cbuildbot_config.py b/buildbot/cbuildbot_config.py
index b51940b..0222c3d 100644
--- a/buildbot/cbuildbot_config.py
+++ b/buildbot/cbuildbot_config.py
@@ -489,7 +489,7 @@
 
   'uprev' : True,
   'overlays': 'both',
-  'push_overlays': None,
+  'push_overlays': 'both',
   'build_tests' : False,
   'unittests' : False,
   'vm_tests' : False,