update_hooks: add a hook to unmerge old and conflicting cross headers

BUG=none
TEST=./update_chroot

Change-Id: Id67b2f486821558d72a73445621f4685a0cf5a54
Reviewed-on: https://gerrit.chromium.org/gerrit/24235
Tested-by: Zdenek Behan <zbehan@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Commit-Ready: Zdenek Behan <zbehan@chromium.org>
1 file changed