Use the update_payload in update_engine

Currently there are two version of the (almost) same code in
  platform/dev/host/lib/update_payload
and
  aosp/system/update_engine/scripts

This patch deletes all the update_payload scripts from the platform/dev and changes all the references to src/aosp/system/update_engine/scripts/update_payload.

BUG=chromium:737210
TEST=cbuildbot --remote
CQ-DEPEND=CL:598435,CL:598435

Change-Id: I3cee23aa3d5ace826d2d6ba669f3bbc21f0a5ac1
Reviewed-on: https://chromium-review.googlesource.com/598434
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Xixuan Wu <xixuan@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/autoupdate.py b/autoupdate.py
index fc16a1c..552b5c2 100644
--- a/autoupdate.py
+++ b/autoupdate.py
@@ -20,17 +20,20 @@
 
 import cherrypy
 
-# Allow importing from dev/host/lib when running from source tree.
-lib_dir = os.path.join(os.path.dirname(__file__), 'host', 'lib')
-if os.path.exists(lib_dir) and os.path.isdir(lib_dir):
-  sys.path.insert(1, lib_dir)
-
 import build_util
 import autoupdate_lib
 import common_util
 import devserver_constants as constants
 import log_util
 # pylint: disable=F0401
+
+# Allow importing from aosp/system/update_engine/scripts when running from
+# source tree. Used for importing update_payload if it is not in the system
+# path.
+lib_dir = os.path.join(os.path.dirname(__file__), '..', '..', 'aosp',
+                       'system', 'update_engine', 'scripts')
+if os.path.exists(lib_dir) and os.path.isdir(lib_dir):
+  sys.path.insert(1, lib_dir)
 import update_payload