Add a method to get the GBB flags of the current firmware.

This method will be used on firmware_DevScreenTimeout test to differentiate
normal timeout and short timeout on developer screen.

BUG=chromium-os:19710
TEST=run_remote_tests.sh --remote=$REMOTE_IP -a "xml_config=$OVERLAY_XML \
         servo_vid=0x18d1 servo_pid=0x5001" firmware_DevScreenTimeout

Change-Id: I2dec1f89017affaa43fdc4efc9db0db03c367a2c
Reviewed-on: https://gerrit.chromium.org/gerrit/13549
Reviewed-by: Rajesh Chenna <rchenna@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Commit-Ready: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>