vapier@: What is the correct procedure to remove imgdiff from building and going into images once these two CL's landed? imgdiff is not used by anyone anymore.

https://chromium-review.googlesource.com/c/602733
https://chromium-review.googlesource.com/c/602676

Sorry, imgdiff does not go into device images, but it sill builds and I don't think it is needed anymore unless you just want to keep it there which is fine too.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/feebccb55dd42f17245a5bfe472214574503eb94

commit feebccb55dd42f17245a5bfe472214574503eb94
Author: Amin Hassani <ahassani@google.com>
Date: Sat Aug 12 01:07:04 2017

update_engine: Remove dependency to imgdiff

We don't use imgdiff anymore. This CL removes update_engine's dependency
to imgdiff.

BUG= chromium:752635 
TEST=./build_image --board=amd64-generic
CQ-DEPEND=CL:602733

Change-Id: I41ca4fe93c06ceca07ad5fcbbaa9dba0d53dda47
Reviewed-on: https://chromium-review.googlesource.com/602676
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

[modify] https://crrev.com/feebccb55dd42f17245a5bfe472214574503eb94/chromeos-base/update_engine/update_engine-9999.ebuild