faft: Don't need the USB check after reimaging to USB

Because the reimaging proces confirms the USB is presented and is a test
image. So don't need the similar USB check anymore. It reduces 30 seconds
per test.

BUG=chromium-os:34392
TEST=run the following test with reimaging, see no USB check later:
$ ./run_remote_tests.sh --board daisy --remote dut -a \
    "image=/path/to/chromiumos_test_image.bin" RecoveryButton/control$

Change-Id: I9fc4e145ed1b558336731354af889f7e1474f482
Reviewed-on: https://gerrit.chromium.org/gerrit/36312
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 ddcc53a..9f96a71 100644
--- a/server/cros/faftsequence.py
+++ b/server/cros/faftsequence.py
@@ -476,6 +476,8 @@
         self.run_faft_sequence()
         # 'Unplug' any USB keys in the servo from the dut.
         self.servo.enable_usb_hub(host=True)
+        # Mark usb_check done so it won't check a test image in USB anymore.
+        self.mark_setup_done('usb_check')
 
 
     def clear_set_gbb_flags(self, clear_mask, set_mask):