sys/net/uhcp: make use of gnrc_netif_ipv6_add_prefix()
sys/net/uhcp: make use of gnrc_netif_ipv6_add_prefix()

OlegHahm/RIOT,miri64/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kYc0o/RIOT,kaspar030/RIOT,jasonatran/RIOT,authmillenon/RIOT,miri64/RIOT,kaspar030/RIOT,jasonatran/RIOT,ant9000/RIOT,jasonatran/RIOT,ant9000/RIOT,authmillenon/RIOT,kYc0o/RIOT,kaspar030/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,miri64/RIOT,OlegHahm/RIOT,ant9000/RIOT,authmillenon/RIOT,kYc0o/RIOT,authmillenon/RIOT,miri64/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,miri64/RIOT,kYc0o/RIOT,ant9000/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,authmillenon/RIOT,jasonatran/RIOT