gnrc_ipv6_nib: implement behavior for router discovery

sys/include/net/gnrc/ipv6/nib.h

sys/include/net/gnrc/ipv6/nib/conf.h

sys/include/net/gnrc/netif2/ipv6.h

sys/net/gnrc/application_layer/uhcpc/gnrc_uhcpc.c

sys/net/gnrc/netif2/gnrc_netif2.c

sys/net/gnrc/network_layer/ipv6/gnrc_ipv6.c

sys/net/gnrc/network_layer/ipv6/nib/_nib-6ln.c

sys/net/gnrc/network_layer/ipv6/nib/_nib-6ln.h

sys/net/gnrc/network_layer/ipv6/nib/_nib-6lr.c

sys/net/gnrc/network_layer/ipv6/nib/_nib-6lr.h

sys/net/gnrc/network_layer/ipv6/nib/_nib-arsm.c

sys/net/gnrc/network_layer/ipv6/nib/_nib-internal.c

sys/net/gnrc/network_layer/ipv6/nib/_nib-internal.h

sys/net/gnrc/network_layer/ipv6/nib/_nib-router.c

sys/net/gnrc/network_layer/ipv6/nib/_nib-router.h

sys/net/gnrc/network_layer/ipv6/nib/nib.c

sys/net/gnrc/network_layer/ipv6/nib/nib_pl.c