make_conf_util: Include chromeos-partner-overlay in PORTDIR_OVERLAY.

We are introducing private SDK packages in chromeos-partner-overlay.

BUG=chromium:890672
TEST=./update_chroot
TEST=portageq envvar PORTDIR_OVERLAY

Change-Id: Ia700a58d3ec7dd919c0522b13ad6d94365d582a7
Reviewed-on: https://chromium-review.googlesource.com/1307277
Commit-Ready: Shuhei Takahashi <nya@chromium.org>
Tested-by: Shuhei Takahashi <nya@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed