tmpfiles: create /etc/resolv.conf symlink only on boot

We will create the symlink on boot as a fallback to provide name
resolution. But if the symlink was removed afterwards, it most likely
should not be recreated. Creating it only on boot also solves the
issue where it would be created prematurely during installation,
before the system was actually booted.

https://bugzilla.redhat.com/show_bug.cgi?id=1197204