2/3 Allow devserver RPC to use arbitrary payload file for update.

This add support for the payload_filename to the cros_au RPC and transports it
to chromite/lib/auto_updater.py.

BUG=chromium:709710
TEST=autoupdate_EndToEndTest

Change-Id: I22657038f57d84ba46c65388d5fe3c21759ab72a
Reviewed-on: https://chromium-review.googlesource.com/471882
Commit-Ready: Hsinyu Chao <hychao@chromium.org>
Tested-by: David Haddock <dhaddock@chromium.org>
Reviewed-by: Xixuan Wu <xixuan@chromium.org>
diff --git a/devserver.py b/devserver.py
index b923090..fafeb53 100755
--- a/devserver.py
+++ b/devserver.py
@@ -909,6 +909,7 @@
     full_update = _parse_boolean_arg(kwargs, 'full_update')
     async = _parse_boolean_arg(kwargs, 'async')
     original_build = _parse_string_arg(kwargs, 'original_build')
+    payload_filename = _parse_string_arg(kwargs, 'payload_filename')
 
     if async:
       path = os.path.dirname(os.path.abspath(__file__))
@@ -932,6 +933,9 @@
       if full_update:
         args = ('%s --full_update' % args)
 
+      if payload_filename:
+        args = ('%s --payload_filename %s' % (args, payload_filename))
+
       p = subprocess.Popen([args], shell=True, preexec_fn=os.setsid)
       pid = os.getpgid(p.pid)