fixed the exception of close socket and added dhcp and dns demo

Ubiquitous/XiUOS/resources/ethernet/LwIP/api/api_lib.c

@@ -246,7 +246,7 @@ netconn_delete(struct netconn *conn)
   } else
 #endif /* LWIP_NETCONN_FULLDUPLEX */
   {
-    err = netconn_prepare_delete(conn);
+//    err = netconn_prepare_delete(conn);
   }
   if (err == ERR_OK) {
     netconn_free(conn);

Ubiquitous/XiUOS/resources/ethernet/LwIP/api/tcpip.c

@@ -601,8 +601,14 @@ tcpip_callbackmsg_trycallback_fromisr(struct tcpip_callback_msg *msg)
 void
 tcpip_init(tcpip_init_done_fn initfunc, void *arg)
 {
+  static int tcpip_flag = 0;
   lwip_init();
 
+  if(tcpip_flag)
+    return;
+
+  tcpip_flag = 1;
+
   tcpip_init_done = initfunc;
   tcpip_init_done_arg = arg;
   if (sys_mbox_new(&tcpip_mbox, TCPIP_MBOX_SIZE) != ERR_OK) {