net: Move PING out of net.c

Separate this functionality out of the net.c behemoth

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/net/Makefile b/net/Makefile
index 0916a56..f1c4859 100644
--- a/net/Makefile
+++ b/net/Makefile
@@ -35,6 +35,7 @@
 COBJS-$(CONFIG_CMD_NET)  += net.o
 COBJS-$(CONFIG_BOOTP_RANDOM_DELAY) += net_rand.o
 COBJS-$(CONFIG_CMD_NFS)  += nfs.o
+COBJS-$(CONFIG_CMD_PING) += ping.o
 COBJS-$(CONFIG_CMD_RARP) += rarp.o
 COBJS-$(CONFIG_CMD_SNTP) += sntp.o
 COBJS-$(CONFIG_CMD_NET)  += tftp.o