net: lwip: add DHCP support and dhcp commmand

Add what it takes to enable NETDEVICES with NET_LWIP and enable DHCP as
well as the dhcp command. CMD_TFTPBOOT is selected by BOOTMETH_EFI due
to this code having an implicit dependency on do_tftpb().

Note that PXE is likely non-fonctional with NET_LWIP (or at least not
100% functional) because DHCP option 209 is not supported by the lwIP
library. Therefore, BOOTP_PXE_DHCP_OPTION cannot be enabled.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Tested-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/net/lwip/Makefile b/net/lwip/Makefile
new file mode 100644
index 0000000..4e92a10
--- /dev/null
+++ b/net/lwip/Makefile
@@ -0,0 +1,5 @@
+ccflags-y += -I$(srctree)/lib/lwip/lwip/src/include -I$(srctree)/lib/lwip/u-boot
+
+obj-$(CONFIG_$(SPL_)DM_ETH) += net-lwip.o
+obj-$(CONFIG_CMD_DHCP) += dhcp.o
+obj-$(CONFIG_CMD_TFTPBOOT) += tftp.o