cmd: simplify network definitions in Makefile

/Makefile already adds lib include paths to UBOOTINCLUDE. There is no point
in adding the same paths again.

Clearly separate the lines relating to NET and to NET_LWIP.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/cmd/Makefile b/cmd/Makefile
index b4668eb..d1f369d 100644
--- a/cmd/Makefile
+++ b/cmd/Makefile
@@ -127,13 +127,10 @@
 endif
 obj-$(CONFIG_CMD_MUX) += mux.o
 obj-$(CONFIG_CMD_NAND) += nand.o
-ifdef CONFIG_CMD_NET
-obj-$(CONFIG_NET) += net.o
-obj-$(CONFIG_NET_LWIP) += net-lwip.o
-obj-$(filter y,$(CONFIG_CMD_NET) $(CONFIG_CMD_NET_LWIP)) += net-common.o
-lwip-includes := -I$(srctree)/lib/lwip/lwip/src/include -I$(srctree)/lib/lwip/u-boot
-CFLAGS_net-lwip.o := $(lwip-includes)
-CFLAGS_net-common.o := $(lwip-includes)
+ifdef CONFIG_NET
+obj-$(CONFIG_CMD_NET) += net.o net-common.o
+else ifdef CONFIG_NET_LWIP
+obj-$(CONFIG_CMD_NET) += net-lwip.o net-common.o
 endif
 obj-$(CONFIG_ENV_SUPPORT) += nvedit.o
 obj-$(CONFIG_CMD_NVEDIT_EFI) += nvedit_efi.o