rarp: Condtionally compile rarp support

Most people don't use the 'rarpboot' command, so only enable it when
CONFIG_CMD_RARP is defined.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>
diff --git a/net/Makefile b/net/Makefile
index 4f819dd..216d1ec 100644
--- a/net/Makefile
+++ b/net/Makefile
@@ -32,7 +32,7 @@
 COBJS-$(CONFIG_CMD_NET)  += eth.o
 COBJS-$(CONFIG_CMD_NET)  += net.o
 COBJS-$(CONFIG_CMD_NFS)  += nfs.o
-COBJS-$(CONFIG_CMD_NET)  += rarp.o
+COBJS-$(CONFIG_CMD_RARP) += rarp.o
 COBJS-$(CONFIG_CMD_SNTP) += sntp.o
 COBJS-$(CONFIG_CMD_NET)  += tftp.o