faft: Plug out USB disk when switching from normal to dev mode

Because plugging-in an USB disk will boot to recovery image without any
warning, that makes it unable to switch to dev mode via the recovery screen.

BUG=chrome-os-partner:11694,chromium-os:32971
TEST=DUT in normal mode, run firmware_TryFwB/control.dev passed.

Change-Id: Ic02bf4a0f67ff314da56733c663fbf081cade911
Reviewed-on: https://gerrit.chromium.org/gerrit/28462
Reviewed-by: Mike Truty <truty@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 b6e04ed..f2d9c91 100644
--- a/server/cros/faftsequence.py
+++ b/server/cros/faftsequence.py
@@ -749,6 +749,8 @@
 
     def enable_keyboard_dev_mode(self):
         logging.info("Enabling keyboard controlled developer mode")
+        # Plug out USB disk for preventing recovery boot without warning
+        self.servo.set('usb_mux_sel1', 'servo_sees_usbkey')
         # Rebooting EC with rec mode on. Should power on AP.
         self.servo.enable_recovery_mode()
         self.servo.cold_reset()