FAFT: Modify the GBB flags directly on the image

Don't call the set_gbb_flags script which may not exists on all
boards. Modify the GBB flags directly on the raw firmware image.

BUG=chromium:527484
TEST=Ran a FAFT test to verify the GBB flags changed.

Change-Id: Ibf6aef9ce19e17b2d132c75c0bb5ea5b762d87bb
Reviewed-on: https://chromium-review.googlesource.com/295770
Commit-Ready: Wai-Hong Tam <waihong@chromium.org>
Tested-by: Wai-Hong Tam <waihong@chromium.org>
Reviewed-by: danny chan <dchan@chromium.org>
diff --git a/client/cros/faft/rpc_functions.py b/client/cros/faft/rpc_functions.py
index 8f1eae3..4c40696 100755
--- a/client/cros/faft/rpc_functions.py
+++ b/client/cros/faft/rpc_functions.py
@@ -304,6 +304,13 @@
         """
         return self._bios_handler.get_gbb_flags()
 
+    def _bios_set_gbb_flags(self, flags):
+        """Set the GBB flags.
+
+        @param flags: An integer of the GBB flags.
+        """
+        self._bios_handler.set_gbb_flags(flags, write_through=True)
+
     def _bios_get_preamble_flags(self, section):
         """Get the preamble flags of a firmware section.