lwip: move net/lwip/ping.c to cmd/lwip

Prepare to split the ping command from cmd/net-lwip.c by moving the
implementation from net/lwip/dns.c to cmd/lwip.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/cmd/lwip/Makefile b/cmd/lwip/Makefile
index 6c4ab58..da79b64 100644
--- a/cmd/lwip/Makefile
+++ b/cmd/lwip/Makefile
@@ -1 +1,2 @@
 obj-$(CONFIG_CMD_DNS) += dns.o
+obj-$(CONFIG_CMD_PING) += ping.o
diff --git a/net/lwip/ping.c b/cmd/lwip/ping.c
similarity index 100%
rename from net/lwip/ping.c
rename to cmd/lwip/ping.c
diff --git a/net/lwip/Makefile b/net/lwip/Makefile
index 255c7d0..97299d9 100644
--- a/net/lwip/Makefile
+++ b/net/lwip/Makefile
@@ -2,7 +2,6 @@
 
 obj-$(CONFIG_$(PHASE_)DM_ETH) += net-lwip.o
 obj-$(CONFIG_CMD_DHCP) += dhcp.o
-obj-$(CONFIG_CMD_PING) += ping.o
 obj-$(CONFIG_CMD_TFTPBOOT) += tftp.o
 obj-$(CONFIG_WGET) += wget.o