ng_ipv6_netif: use EUI-64 as base for 6LoWPAN IPv6 addresses

[RFC 6775](https://tools.ietf.org/html/rfc6775#section-1.3) suggests
strongly to use the EUI-64 as basis for the IPv6 addresses, since
otherwise duplicate address detection and some measures to give new
addresses (in case of duplicate) e.g. DHCPv6 needs to be in place.