6lowpan nd: send rtr adv after receiving a rtr adv

A router should turn into a router after receiving a valid router advertisement.

sys/net/gnrc/network_layer/ndp/gnrc_ndp.c

@@ -613,6 +613,9 @@ void gnrc_ndp_rtr_adv_handle(kernel_pid_t iface, gnrc_pktsnip_t *pkt, ipv6_hdr_t
         gnrc_sixlowpan_nd_rtr_sol_reschedule(nc_entry, next_rtr_sol);
         gnrc_ndp_internal_send_nbr_sol(nc_entry->iface, NULL, &nc_entry->ipv6_addr,
                                        &nc_entry->ipv6_addr);
+        if (if_entry->flags & GNRC_IPV6_NETIF_FLAGS_ROUTER) {
+            gnrc_ipv6_netif_set_rtr_adv(if_entry, true);
+        }
     }
 #endif
 }