Do not merge to AOSP: Fixes the link to brillo-clang-format in CrOS

Currently the .clang-format file in update_engine points to
../../build/tools/brillo-clang-format which is a path in AOSP source
code workflow and doesn't exits in CrOS side. This CL changes that
softlink to ../../../platform2/common-mk/brillo-clang-format.

BUG=none
TEST=clang-format -i ./*; git clang-format

Change-Id: I95bdae053b58ee1b43d533f8eb03615f46fa33b4
Reviewed-on: https://chromium-review.googlesource.com/549982


Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>