configs/aarch64_efi: support net boot with devicetree

UEFI platforms with a Devicetree have recently gained support for some form
of network booting. [1][2]

On those platforms, the UEFI firmware downloads the disk image over the
network and presents it to UEFI applications using the simple filesystem
protocol. This is sufficient for OS loaders such as e.g. GRUB to find their
configuration and load the kernel image.

The firmware must also describe the ramdisk to the OS, so that it finds its
root filesystem. On ACPI based platforms this is done with an NVDIMM
Firmware Interface Table (NFIT). On Devicetree based platforms, this can be
done with a pmem node. [3]

Add a kernel config fragment to add pmem support, which enables network
boot on UEFI platforms with Devicetree.
Also, briefly mention that we support this scenario in the readme.

This can be tested on Qemu, with the following procedure:

Build aarch64_efi_defconfig in a folder, to obtain output/images/disk.img,
and serve the image over HTTP with python:

  $ python -m http.server -d output/images/

In another terminal and another folder, configure a U-Boot based firmware
starting from qemu_aarch64_ebbr_defconfig, and with the following
additional configurations:

  BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2026.04"
  BR2_TARGET_UBOOT_NEEDS_DTC=y

Add the following U-Boot configurations to
board/qemu/aarch64-ebbr/u-boot.fragment:

  CONFIG_EFI_HTTP_BOOT=y
  CONFIG_NET_LWIP=y

Build everything; you should obtain output/images/flash.bin.

Start Qemu as per board/qemu/aarch64-ebbr/readme.txt, but omitting the hd0
-device and -drive stanzas.
U-Boot should start; interrupt it with enter.

At U-Boot prompt, type the following commands:

  => setenv loadaddr 0x43000000
  => efidebug boot rm 0
  => efidebug boot add -u 0 net http://10.0.2.2:8000/disk.img
  => efidebug boot order 0
  => bootefi bootmgr

U-Boot should download the disk image over HTTP and boot Linux with UEFI.
Look for the following message in Linux boot log, to confirm that it did
indeed mount its rootfs from the downloaded image:

  EXT4-fs (pmem0p2): mounted filesystem ...

Link: https://www.linaro.org/blog/installing-fedora-with-uefi-http-boot/ [1]
Link: https://docs.u-boot-project.org/en/latest/develop/uefi/uefi.html#uefi-http-boot-using-the-legacy-tcp-stack [2]
Link: https://github.com/ARM-software/edge-iot-arch-guide/blob/main/source/http-boot/pmem_node.md [3]
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Dick Olsson <hi@senzilla.io>
Signed-off-by: Julien Olivain <ju.o@free.fr>

configs/aarch64_efi_defconfig

@@ -11,6 +11,7 @@ BR2_LINUX_KERNEL=y
 BR2_LINUX_KERNEL_CUSTOM_VERSION=y
 BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="6.18.14"
 BR2_LINUX_KERNEL_USE_ARCH_DEFAULT_CONFIG=y
+BR2_LINUX_KERNEL_CONFIG_FRAGMENT_FILES="board/aarch64-efi/linux-pmem-netboot.fragment"
 BR2_LINUX_KERNEL_NEEDS_HOST_OPENSSL=y
 BR2_LINUX_KERNEL_NEEDS_HOST_PYTHON3=y
 BR2_TARGET_ROOTFS_EXT2=y