Ensure push_image is True for firmware branches.

BUG=None
TEST=None

Change-Id: Idd3e3fe1669fd6c8f84d811592cae2fcc2f761f9
Reviewed-on: https://gerrit.chromium.org/gerrit/28889
Reviewed-by: David James <davidjames@chromium.org>
Commit-Ready: David McMahon <djmm@chromium.org>
Reviewed-by: David McMahon <djmm@chromium.org>
Tested-by: David McMahon <djmm@chromium.org>
diff --git a/buildbot/cbuildbot_config.py b/buildbot/cbuildbot_config.py
index c1c0f92..e5a76d4 100755
--- a/buildbot/cbuildbot_config.py
+++ b/buildbot/cbuildbot_config.py
@@ -975,7 +975,6 @@
 
 _firmware_release = _release.derive(
   images=['base'],
-  push_image=False,
   build_tests=False,
   unittests=False,
   vm_tests=None,