fff-gateway: add package

This introduces a new script for simple gateway configuration.

The main configuregateway script is able to execute functions
for various steps like 'configure' or 'apply' from scripts in /etc/gateway.d.

This makes it easy to distribute configuration to the appropriate packages.

Signed-off-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>
Reviewed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>