feat(lwip):add fall back dns config in menuconfig

Author: freakyxue

src/core/dns.c

@@ -325,7 +325,7 @@ dns_init(void)
   /* initialize fallback dns DNS server address */
   ip_addr_t dnsserver;
   FALLBACK_DNS_SERVER_ADDRESS(&dnsserver);
-#if LWIP_IPV4 && LWIP_IPV6
+#if LWIP_IPV4 && LWIP_IPV6 && ESP_LWIP_FALLBACK_DNS_PREFER_IPV4
   dnsserver.type = IPADDR_TYPE_V4;
 #endif
   dns_setserver(DNS_FALLBACK_SERVER_INDEX, &dnsserver);