lwip: split cmd/net-lwip.c into one file per command

Move each command in cmd/net-lwip.c into its own file
(cmd/lwip/${cmd}.c).

Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/cmd/lwip/ping.c b/cmd/lwip/ping.c
index 1390b91..6996931 100644
--- a/cmd/lwip/ping.c
+++ b/cmd/lwip/ping.c
@@ -13,6 +13,9 @@
 #include <net.h>
 #include <time.h>
 
+U_BOOT_CMD(ping, 2, 1, do_ping, "send ICMP ECHO_REQUEST to network host",
+	   "pingAddress");
+
 #define PING_DELAY_MS 1000
 #define PING_COUNT 5
 /* Ping identifier - must fit on a u16_t */