Move USB probing logic and recovery image install to servo.py.

BUG=chrome-os-partner:381
TEST=Run manually

Change-Id: I2211b7a4b24fff122e3ff6357534a89b4ab0a3ff
Reviewed-on: https://gerrit.chromium.org/gerrit/12258
Reviewed-by: Jon Salz <jsalz@chromium.org>
Tested-by: Jon Salz <jsalz@chromium.org>
Commit-Ready: Jon Salz <jsalz@chromium.org>
diff --git a/server/cros/faftsequence.py b/server/cros/faftsequence.py
index cf3ec05..da65529 100644
--- a/server/cros/faftsequence.py
+++ b/server/cros/faftsequence.py
@@ -106,7 +106,7 @@
           error.TestError: if USB disk not detected or not a test image.
         """
         self.servo.set('usb_mux_sel1', 'servo_sees_usbkey')
-        usb_dev = self.probe_host_usb_dev()
+        usb_dev = self.servo.probe_host_usb_dev()
         if not usb_dev:
             raise error.TestError(
                     'An USB disk should be plugged in the servo board.')