gateway.d: Add configuration script for meta information

This adds a configuration script for applying hostname,
coordinates, hoodname and contact address.

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