Add chroot hook to unmerge old libchromeos

In order to eliminate file conflicts between old libchromeos and new
libbrillo packages, unmerge the old one before compiling/installing
the new one.

BUG=b:25345697
TEST=./update_chroot
CQ-DEPEND=CL:309584

Change-Id: Ifa9e322b2d9657f26d218299da362b78aa11e1b6
Reviewed-on: https://chromium-review.googlesource.com/309820
Commit-Ready: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed