net: lwip: build lwIP

Build the lwIP library when NET_LWIP is enabled. The following files
are adaptation layers written specially for U-Boot:

 lib/lwip/u-boot/arch/cc.h
 lib/lwip/u-boot/arch/sys_arch.h (empty)
 lib/lwip/u-boot/limits.h (empty)
 lib/lwip/u-boot/lwipopts.h

They were initially contributed by Maxim in a previous RFC patch series.

The lwIP stack needs to be added as a Git subtree with the following
command:

 $ git subtree add --squash --prefix lib/lwip/lwip \
   https://github.com/lwip-tcpip/lwip.git  STABLE-2_2_0_RELEASE

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Co-developed-by: Maxim Uvarov <muvarov@gmail.com>
Cc: Maxim Uvarov <muvarov@gmail.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/lib/lwip/Makefile b/lib/lwip/Makefile
new file mode 100644
index 0000000..dfcd700
--- /dev/null
+++ b/lib/lwip/Makefile
@@ -0,0 +1,55 @@
+# SPDX-License-Identifier: GPL-2.0+
+#
+# Copyright (C) 2024 Linaro Ltd.
+
+obj-y += \
+	lwip/src/api/api_lib.o \
+	lwip/src/api/api_msg.o \
+	lwip/src/api/err.o \
+	lwip/src/api/if_api.o \
+	lwip/src/api/netbuf.o \
+	lwip/src/api/netdb.o \
+	lwip/src/api/netifapi.o \
+	lwip/src/api/sockets.o \
+	lwip/src/api/tcpip.o \
+	lwip/src/apps/http/http_client.o \
+	lwip/src/apps/tftp/tftp.o \
+	lwip/src/core/altcp_alloc.o \
+	lwip/src/core/altcp.o \
+	lwip/src/core/altcp_tcp.o \
+	lwip/src/core/def.o \
+	lwip/src/core/dns.o \
+	lwip/src/core/inet_chksum.o \
+	lwip/src/core/init.o \
+	lwip/src/core/ip.o \
+	lwip/src/core/ipv4/acd.o \
+	lwip/src/core/ipv4/autoip.o \
+	lwip/src/core/ipv4/dhcp.o \
+	lwip/src/core/ipv4/etharp.o \
+	lwip/src/core/ipv4/icmp.o \
+	lwip/src/core/ipv4/igmp.o \
+	lwip/src/core/ipv4/ip4_addr.o \
+	lwip/src/core/ipv4/ip4.o \
+	lwip/src/core/ipv4/ip4_frag.o \
+	lwip/src/core/ipv6/dhcp6.o \
+	lwip/src/core/ipv6/ethip6.o \
+	lwip/src/core/ipv6/icmp6.o \
+	lwip/src/core/ipv6/inet6.o \
+	lwip/src/core/ipv6/ip6_addr.o \
+	lwip/src/core/ipv6/ip6.o \
+	lwip/src/core/ipv6/ip6_frag.o \
+	lwip/src/core/ipv6/mld6.o \
+	lwip/src/core/ipv6/nd6.o \
+	lwip/src/core/mem.o \
+	lwip/src/core/memp.o \
+	lwip/src/core/netif.o \
+	lwip/src/core/pbuf.o \
+	lwip/src/core/raw.o \
+	lwip/src/core/stats.o \
+	lwip/src/core/sys.o \
+	lwip/src/core/tcp.o \
+	lwip/src/core/tcp_in.o \
+	lwip/src/core/tcp_out.o \
+	lwip/src/core/timeouts.o \
+	lwip/src/core/udp.o \
+	lwip/src/netif/ethernet.o