update_engine: Change imgdiff/imgpatch to a dummy puffdiff/puffpatch

Currently we never create imgdiff operation as it was never
activated. In near future we will be adding puffdiff and puffpatch
operations which replaces the imgdiff. This CL does not actually adds
puffdiff, but adds a placeholder for it.

BUG=none
TEST=cros_workon_make --board=amd64-generic --test update_engine

Change-Id: I6453048acb65c052354a1658f0c6fd41ad33e242
Reviewed-on: https://chromium-review.googlesource.com/602733
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>

scripts/update_payload/common.py

diff --git a/scripts/update_payload/common.py b/scripts/update_payload/common.py
index 678fc5d..bab8a4f 100644
--- a/scripts/update_payload/common.py
+++ b/scripts/update_payload/common.py
@@ -27,7 +27,7 @@
 INPLACE_MINOR_PAYLOAD_VERSION = 1
 SOURCE_MINOR_PAYLOAD_VERSION = 2
 OPSRCHASH_MINOR_PAYLOAD_VERSION = 3
-IMGDIFF_MINOR_PAYLOAD_VERSION = 4
+PUFFDIFF_MINOR_PAYLOAD_VERSION = 4
 
 #
 # Payload operation types.
@@ -45,9 +45,9 @@
   ZERO = _CLASS.ZERO
   DISCARD = _CLASS.DISCARD
   REPLACE_XZ = _CLASS.REPLACE_XZ
-  IMGDIFF = _CLASS.IMGDIFF
+  PUFFDIFF = _CLASS.PUFFDIFF
   ALL = (REPLACE, REPLACE_BZ, MOVE, BSDIFF, SOURCE_COPY, SOURCE_BSDIFF, ZERO,
-         DISCARD, REPLACE_XZ, IMGDIFF)
+         DISCARD, REPLACE_XZ, PUFFDIFF)
   NAMES = {
       REPLACE: 'REPLACE',
       REPLACE_BZ: 'REPLACE_BZ',
@@ -58,7 +58,7 @@
       ZERO: 'ZERO',
       DISCARD: 'DISCARD',
       REPLACE_XZ: 'REPLACE_XZ',
-      IMGDIFF: 'IMGDIFF',
+      PUFFDIFF: 'PUFFDIFF',
   }
 
   def __init__(self):