cbuildbot: add a config knob to disable SyncChromeStage

The current heuristics get the answer wrong for some configs (namely the
firmware), so add a config knob to be explicit.

BUG=chromium-os:36324
TEST=`cbuildbot link-firmware` no longer tries to sync chrome
TEST=`./buildbot/run_tests` passes

Change-Id: Ib90871cfea5051dd94ede087da3081605314f0ac
Reviewed-on: https://gerrit.chromium.org/gerrit/44702
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index d0210db..7c203fa 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -608,10 +608,13 @@
     options.chrome_rev = constants.CHROME_REV_SPEC
 
   # If it's likely we'll need to build Chrome, fetch the source.
-  options.managed_chrome = (chrome_rev != constants.CHROME_REV_LOCAL and
-      (not build_config['usepkg_build_packages'] or chrome_rev or
-       build_config['useflags'] or build_config['profile'] or
-       options.rietveld_patches))
+  if build_config['sync_chrome'] is None:
+    options.managed_chrome = (chrome_rev != constants.CHROME_REV_LOCAL and
+        (not build_config['usepkg_build_packages'] or chrome_rev or
+         build_config['useflags'] or build_config['profile'] or
+         options.rietveld_patches))
+  else:
+    options.managed_chrome = build_config['sync_chrome']
 
   if options.managed_chrome:
     # Tell Chrome to fetch the source locally.