systemd: add compatibility symlink for resolv.conf

In 215 resolv.conf moved from /run/systemd/network to
/run/systemd/resolv but there isn't anything to fix references to the
old location (namely existing /etc/resolv.conf symlinks after upgrades).
Adding this rule ensures that those links or any other references
continue to work as they did before.

Backport of https://github.com/coreos/coreos-overlay/pull/745