packages/fff: add new fff-dhcp package

Signed-off-by: Tim Niemeyer <tim@tn-x.org>
Signed-off-by: Fabian Bläse <fabian@blaese.de>
- configure router advertisements
- fix package title
- remove unnecessary dhcp config on first boot
- lower lease time
- only commit dhcp uci section
Acked-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>