[Bugfix]Update netdev info after registered

1.修复ifconfig命令无法正确显示IP，网关，掩码的问题
2.lwip_2.1.0，增加netdev->mtu = lwip_netif->mtu;

components/net/lwip-2.1.0/src/netif/ethernetif.c

@@ -315,6 +315,7 @@ static int netdev_add(struct netif *lwip_netif)
 	
     /* Update netdev info after registered */
     netdev->flags = lwip_netif->flags;
+    netdev->mtu = lwip_netif->mtu;
     netdev->ops = &lwip_netdev_ops;
     netdev->hwaddr_len =  lwip_netif->hwaddr_len;
     rt_memcpy(netdev->hwaddr, lwip_netif->hwaddr, lwip_netif->hwaddr_len);