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>

cmd/lwip/Makefile

@@ -1 +1,2 @@
 obj-$(CONFIG_CMD_DNS) += dns.o
+obj-$(CONFIG_CMD_PING) += ping.o

net/lwip/Makefile

@@ -2,7 +2,6 @@ ccflags-y += -I$(srctree)/lib/lwip/lwip/src/include -I$(srctree)/lib/lwip/u-boot
 
 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