lwip: add net_lwip_dns_resolve()

Add a helper fonction to convert an IP address (supplied as a text
string) or a host name to an ip_addr_t.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/include/net-lwip.h b/include/net-lwip.h
index 58cd384..f54f234 100644
--- a/include/net-lwip.h
+++ b/include/net-lwip.h
@@ -38,6 +38,7 @@
 void net_lwip_remove_netif(struct netif *netif);
 struct netif *net_lwip_get_netif(void);
 int net_lwip_rx(struct udevice *udev, struct netif *netif);
+int net_lwip_dns_resolve(char *name_or_ip, ip_addr_t *ip);
 
 /**
  * wget_validate_uri() - varidate the uri