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/drivers/net/Kconfig b/drivers/net/Kconfig
index fa6fc1c..89f7411 100644
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -97,7 +97,7 @@
 
 menuconfig NETDEVICES
 	bool "Network device support"
-	depends on NET
+	depends on NET || NET_LWIP
 	select DM_ETH
 	help
 	  You must select Y to enable any network device support