Fix devserver with handling custom labels and add preference to always use cached image.

Change-Id: I701cba5bb9ad91a2574e8b309e8fafda528d7bc5

BUG=7152 5924
TEST=Tested with archive_dir and label set using example from dale.  Also tested with factory options that are archive_build with dir and updater using default options.  Url's are correct in both places

Review URL: http://codereview.chromium.org/3533003
diff --git a/devserver.py b/devserver.py
index eecab04..e4516a8 100644
--- a/devserver.py
+++ b/devserver.py
@@ -93,6 +93,8 @@
   parser.add_option('-t', action='store_true', dest='test_image')
   parser.add_option('-u', '--urlbase', dest='urlbase',
                     help='base URL, other than devserver, for update images.')
+  parser.add_option('--use_cached', action="store_true", default=False,
+                    help='Prefer cached image regardless of timestamps.')
   parser.add_option('--validate_factory_config', action="store_true",
                     dest='validate_factory_config',
                     help='Validate factory config file, then exit.')
@@ -124,7 +126,8 @@
       test_image=options.test_image,
       factory_config_path=options.factory_config,
       client_prefix=options.client_prefix,
-      forced_image=options.image)
+      forced_image=options.image,
+      use_cached=options.use_cached)
 
   if options.factory_config:
      updater.ImportFactoryConfigFile(options.factory_config,