update_engine: Process Omaha response for rollback images

Omaha returns whether the image returned is a rollback in the
_rollback="true" argument. If this is set, the client has to check
whether it's OK to apply the rollback image (policy is specifically
requesting a rollback and verified boot will accept the image based
on its kernel and firmware key versions).

In addition to this, the device has to do a safe powerwash if the
image is a rollback. (We're not supporting rollbacks with partial
or no powerwash yet.)

We're also setting the rollback_happened preference to avoid force
updates happening before the policy is available again.

Chromium CL adding the error code: http://crrev.com/c/1047866

BUG=chromium:840432
TEST='cros_run_unit_tests --board=caroline --packages update_engine'

Change-Id: I1436ca96211b2a8523e78bf83602ef8b6b525570
Reviewed-on: https://chromium-review.googlesource.com/1047610
Commit-Ready: Marton Hunyady <hunyadym@chromium.org>
Tested-by: Marton Hunyady <hunyadym@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>

common/error_code.h

diff --git a/common/error_code.h b/common/error_code.h
index 9e7c71d..c301155 100644
--- a/common/error_code.h
+++ b/common/error_code.h
@@ -77,10 +77,11 @@
   // kPayloadTimestampError = 51,
   kUpdatedButNotActive = 52,
   kNoUpdate = 53,
+  kRollbackNotPossible = 54,
 
   // VERY IMPORTANT! When adding new error codes:
   //
-  // 1) Update tools/metrics/histograms/histograms.xml in Chrome.
+  // 1) Update tools/metrics/histograms/enums.xml in Chrome.
   //
   // 2) Update the assorted switch statements in update_engine which won't
   //    build until this case is added.