xbuddy: Introduce the 'signed' image type

This change makes it possible to flash signed images from cros flash
using the `signed` alias.

BUG=chromium:852017
TEST=cros flash usb:// xbuddy://remote/eve-arcnext/latest-canary/signed
TEST=./xbuddy_unittest.py

Change-Id: I1c4c76ca0dd29845ef8639744745904e209a34eb
Reviewed-on: https://chromium-review.googlesource.com/1097609
Commit-Ready: Luis Hector Chavez <lhchavez@chromium.org>
Tested-by: Luis Hector Chavez <lhchavez@chromium.org>
Reviewed-by: Luis Hector Chavez <lhchavez@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
diff --git a/devserver_constants.py b/devserver_constants.py
index 9a72087..70e8c2a 100644
--- a/devserver_constants.py
+++ b/devserver_constants.py
@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 # Copyright (c) 2013 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
@@ -29,6 +30,7 @@
 FACTORY_IMAGE_FILE = 'factory_test/chromiumos_factory_image.bin'
 FACTORY_SHIM_IMAGE_FILE = 'factory_shim/factory_install_shim.bin'
 RECOVERY_IMAGE_FILE = 'recovery_image.bin'
+SIGNED_IMAGE_FILE = 'signed_image.bin'
 TEST_IMAGE_FILE = 'chromiumos_test_image.bin'
 
 ALL_IMAGES = (