In previous versions of Apertis we were using a modified iptables package
containing custom scripting/systemd unit to load iptables rules at boot.
Debian contains the iptables-persistent package which performs this task.

Use this instead of adding the custom scripts to the new version. Add the
custom rules to an overlay so we don't need to modify the package.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>