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/tftp.c b/net/lwip/tftp.c
new file mode 100644
index 0000000..1fa246f
--- /dev/null
+++ b/net/lwip/tftp.c
@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: GPL-2.0+
+/* Copyright (C) 2024 Linaro Ltd. */
+
+#include <command.h>
+#include <net-lwip.h>
+
+int do_tftpb(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
+{
+	/* Not implemented */
+	return CMD_RET_FAILURE;
+}