iptables: add systemd helper unit to load/restore rules

iptables: add systemd helper unit to load/restore rules

There is currently no way to automatically load iptables rules in OE.
Add a systemd unit file to automatically load rules on network
connection. This is cribbed from the way ArchLinux handles iptables with
some minor modifications for OE.
New rules can be generated directly on the target using:
# iptables-save -f /etc/iptables/iptables.rules
Good documentation for writing rules offline is lacking, but the basics
are explained here:
https://unix.stackexchange.com/q/400163/49405

(From OE-Core rev: 76d3574d17c38d93ba4660bdae5730ac222994d4)

Signed-off-by: Jack Mitchell <jack@embed.me.uk>
Signed-off-by: Diego Rondini <diego.rondini@kynetics.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>