faft: Swap the order of checking GBB flags and reimaging USB

Reimging USB may update the firmware also, when firmware_update=True. The
GBB flags are reset. So we should move the GBB flag check after the firmware
update.

BUG=chromium-os:34392
TEST=run the following test with reimaging and firmware update:
$ ./run_remote_tests.sh --board daisy --remote dut -a \
    "image=/path/to/chromiumos_test_image.bin firmware_update=True" \
    TryFwB/control$

Change-Id: Ibd964788d28b950a182786639e8eb104202e0100
Reviewed-on: https://gerrit.chromium.org/gerrit/36423
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 fc96e34..94d9157 100644
--- a/server/cros/faftsequence.py
+++ b/server/cros/faftsequence.py
@@ -223,10 +223,8 @@
             'reboot_action': (self.sync_and_warm_reboot),
             'firmware_action': (None)
         })
+        self.install_test_image(self._install_image_path, self._firmware_update)
         self.setup_gbb_flags()
-        if self._install_image_path:
-            self.install_test_image(self._install_image_path,
-                                    self._firmware_update)
 
 
     def cleanup(self):
@@ -408,6 +406,9 @@
             image_path: An URL or a path on the host to the test image.
             firmware_update: Also update the firmware after installing.
         """
+        if not image_path:
+            return
+
         if self.check_setup_done('reimage'):
             return