faft: Clear the short delay GBB flag before running DevScreenTimeout FAFT

The short delay GBB flag is disabled in the release firmware. So the
DevScreenTimeout should only check this release case.

Clear this flag if presented before running the test.

BUG=chrome-os-partner:9190
TEST=run firmware_DevScreenTimeout on Link passed.

Change-Id: I4489913eadb9efe81a13121495629cb9316e5c8f
Reviewed-on: https://gerrit.chromium.org/gerrit/28583
Reviewed-by: Vic Yang <victoryang@chromium.org>
Commit-Ready: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>
diff --git a/server/cros/faftsequence.py b/server/cros/faftsequence.py
index 4d639fe..fc9f13c 100644
--- a/server/cros/faftsequence.py
+++ b/server/cros/faftsequence.py
@@ -102,6 +102,9 @@
     # Delay between sending keystroke to firmware
     FIRMWARE_KEY_DELAY = 0.5
 
+    # The developer screen timeouts fit our spec.
+    DEV_SCREEN_TIMEOUT = 30
+
     CHROMEOS_MAGIC = "CHROMEOS"
     CORRUPTED_MAGIC = "CORRUPTD"