faft: Skip checking test image in USB when servod runs in a different host

When servod runs in a different host, the original check doesn't work since
we can't access the servo host. Skip the check temporarily such that it won't
break the tests. Should find a way to work-around it.

BUG=chromium-os:34395
TEST=manual, run the following test using Beaglebone:
$ run_remote_tests.sh --board daisy --remote dut \
                      --args "servo_host=dut-servo" RecoveryButton/control$

Change-Id: Ib0c44ca6eec4d17e00d4f85f3c54bd3b5efdf870
Reviewed-on: https://gerrit.chromium.org/gerrit/36211
Commit-Ready: Tom Wai-Hong Tam <waihong@chromium.org>
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>
diff --git a/server/cros/servo.py b/server/cros/servo.py
index df1beb1..0089824 100644
--- a/server/cros/servo.py
+++ b/server/cros/servo.py
@@ -117,6 +117,7 @@
         self._key_matrix = 0
         self._server = None
         self._connect_servod(servo_host, servo_port)
+        self._is_localhost = (servo_host == 'localhost')
 
 
     def initialize_dut(self, cold_reset=False):
@@ -145,6 +146,15 @@
             self.cold_reset()
 
 
+    def is_localhost(self):
+        """Is the servod hosted locally?
+
+        Returns:
+          True if local hosted; otherwise, False.
+        """
+        return self._is_localhost
+
+
     def power_long_press(self):
         """Simulate a long power button press."""
         # After a long power press, the EC may ignore the next power