net: lwip: add ping command

Add support for the the ping command with NET_LWIP. The implementation
is derived from lwIP's contrib/apps/ping/ping.c.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/include/net-lwip.h b/include/net-lwip.h
index 37744b0..9aa371e 100644
--- a/include/net-lwip.h
+++ b/include/net-lwip.h
@@ -17,5 +17,6 @@
 int net_lwip_rx(struct udevice *udev, struct netif *netif);
 
 int do_dhcp(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[]);
+int do_ping(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[]);
 
 #endif /* __NET_LWIP_H__ */