AU: Verify source rootfs/kernel hashes before applying delta.

New style full updates will not send the old rootfs hash so no check takes
place.

BUG=7562
TEST=unit tests, gmerged on device and tested with good/bad source partition

Change-Id: I65b28bf57110e4d87472d4aea59121878cde24b0

Review URL: http://codereview.chromium.org/3712003
diff --git a/action_processor.h b/action_processor.h
index fc42a24..9bc4ef2 100644
--- a/action_processor.h
+++ b/action_processor.h
@@ -37,6 +37,7 @@
   kActionCodeDownloadSizeMismatchError = 11,
   kActionCodeDownloadPayloadVerificationError = 12,
   kActionCodeDownloadAppliedUpdateVerificationError = 13,
+  kActionCodeDownloadWriteError = 14,
 };
 
 class AbstractAction;