make_chroot: link in new eclass overlay

We need to create a link to the new eclass overlay so our boards
can leverage it.

While we're here, clear out the old /usr/portage one as that repo
no longer exists.

BUG=chromium:205164
TEST=`./update_chroot && ./build_packages --board=stumpy` works

Change-Id: I59e69b5cc586c63648139739de51dace7c7ab6bf
Reviewed-on: https://chromium-review.googlesource.com/180140
Reviewed-by: Chris Masone <cmasone@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
2 files changed