fff-network: Calculate IPv6 using ROUTERMAC variable

If the mac is read from /sys/class/net/${iface}/address, some
devices (WA860RE, Picostation) will not set the fdff addresses.

This can be fixed by using the $ROUTERMAC in configurenetwork.

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