releng: Fix 81-dhcpcd.rules

Commit a526d961 broke this rule for virtio and other
devices where no ID_NET_* is exported.

configs/releng/root-image/etc/udev/rules.d/81-dhcpcd.rules

-ACTION=="add", SUBSYSTEM=="net", NAME=="en*|eth*", ENV{SYSTEMD_WANTS}="dhcpcd@$name.service"
+ACTION=="add", SUBSYSTEM=="net", ENV{INTERFACE}=="en*|eth*", ENV{SYSTEMD_WANTS}="dhcpcd@$name.service"