lwip: ping: accept host name when CONFIG_CMD_DNS=y

Update the ping command so that a host name is accepted in addition to
an IP address, provided DNS support is enabled.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/cmd/lwip/ping.c b/cmd/lwip/ping.c
index 6996931..87f8e95 100644
--- a/cmd/lwip/ping.c
+++ b/cmd/lwip/ping.c
@@ -14,7 +14,7 @@
 #include <time.h>
 
 U_BOOT_CMD(ping, 2, 1, do_ping, "send ICMP ECHO_REQUEST to network host",
-	   "pingAddress");
+	   "pingAddressOrHostName");
 
 #define PING_DELAY_MS 1000
 #define PING_COUNT 5
@@ -167,7 +167,7 @@
 	if (argc < 2)
 		return CMD_RET_USAGE;
 
-	if (!ipaddr_aton(argv[1], &addr))
+	if (net_lwip_dns_resolve(argv[1], &addr))
 		return CMD_RET_USAGE;
 
 restart: