net: convert a makefile to Kbuild style

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/net/Makefile b/net/Makefile
index 4c9a97f..31aadc2 100644
--- a/net/Makefile
+++ b/net/Makefile
@@ -5,39 +5,17 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-include $(TOPDIR)/config.mk
-
 # CFLAGS += -DDEBUG
 
-LIB	= $(obj)libnet.o
-
-COBJS-$(CONFIG_CMD_NET)  += arp.o
-COBJS-$(CONFIG_CMD_NET)  += bootp.o
-COBJS-$(CONFIG_CMD_CDP)  += cdp.o
-COBJS-$(CONFIG_CMD_DNS)  += dns.o
-COBJS-$(CONFIG_CMD_NET)  += eth.o
-COBJS-$(CONFIG_CMD_LINK_LOCAL) += link_local.o
-COBJS-$(CONFIG_CMD_NET)  += net.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
-
-COBJS	:= $(sort $(COBJS-y))
-SRCS	:= $(COBJS:.o=.c)
-OBJS	:= $(addprefix $(obj),$(COBJS))
-
-all:	$(LIB)
-
-$(LIB):	$(obj).depend $(OBJS)
-	$(call cmd_link_o_target, $(OBJS))
-
-#########################################################################
-
-# defines $(obj).depend target
-include $(SRCTREE)/rules.mk
-
-sinclude $(obj).depend
-
-#########################################################################
+obj-$(CONFIG_CMD_NET)  += arp.o
+obj-$(CONFIG_CMD_NET)  += bootp.o
+obj-$(CONFIG_CMD_CDP)  += cdp.o
+obj-$(CONFIG_CMD_DNS)  += dns.o
+obj-$(CONFIG_CMD_NET)  += eth.o
+obj-$(CONFIG_CMD_LINK_LOCAL) += link_local.o
+obj-$(CONFIG_CMD_NET)  += net.o
+obj-$(CONFIG_CMD_NFS)  += nfs.o
+obj-$(CONFIG_CMD_PING) += ping.o
+obj-$(CONFIG_CMD_RARP) += rarp.o
+obj-$(CONFIG_CMD_SNTP) += sntp.o
+obj-$(CONFIG_CMD_NET)  += tftp.o