Merge patch series "Introduce the lwIP network stack"

Jerome Forissier <jerome.forissier@linaro.org> says:

This is a rework of a patch series by Maxim Uvarov: "net/lwip: add lwip
library for the network stack" [1]. The goal is to introduce the lwIP TCP/IP
stack [2] [3] as an alternative to the current implementation in net/,
selectable with Kconfig, and ultimately keep only lwIP if possible. Some
reasons for doing so are:
- Make the support of HTTPS in the wget command easier. Javier T. and
Raymond M. (CC'd) have some additional lwIP and Mbed TLS patches to do
so. With that it becomes possible to fetch and launch a distro installer
such as Debian etc. using a secure, authenticated connection directly
from the U-Boot shell. Several use cases:
  * Authentication: prevent MITM attack (third party replacing the
binary with a different one)
  * Confidentiality: prevent third parties from grabbing a copy of the
image as it is being downloaded
  * Allow connection to servers that do not support plain HTTP anymore
(this is becoming more and more common on the Internet these days)
- Possibly benefit from additional features implemented in lwIP
- Less code to maintain in U-Boot

Prior to applying this 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

Notes

1. A number of features are currently incompatible with NET_LWIP:
DFU_TFTP, FASTBOOT, SPL_NET, ETH_SANDBOX, ETH_SANDBOX_RAW, DM_ETH. They
all make assumptions on how the network stack is implemented and/or
pull sybols that are not trivially exported from lwIP. Some interface
rework may be needed.

2. Due to the above, and in order to provide some level of testing of the
lwIP code in CI even when the legacy NET is the default, a new QEMU
configuration is introduced (qemu_arm64_lwip_defconfig) which is
based on qemu_arm64_defconfig with NET_LWIP and CMD_*_LWIP enabled.
In addition to that, this series has some [TESTING] patches
which make NET_LWIP the default.

[1] https://lore.kernel.org/all/20231127125726.3735-1-maxim.uvarov@linaro.org/
[2] https://www.nongnu.org/lwip/
[3] https://en.wikipedia.org/wiki/LwIP

Link: https://lore.kernel.org/r/cover.1729070678.git.jerome.forissier@linaro.org