fff-network/fff-hoods: Only use MAC address in network lib

Previously, IPv6 addresses were constructed by reading from
the device config file in the function. To have more options,
it is better to use the address itself as parameter.

By this way we can decide what we use for getting the MAC when
calling.

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