Ventoy Compatible ISO

Ventoy is a (GPL-3.0-or-later) multiboot tool that uses GRUB to boot ISO files. Unlike others I've seen, Ventoy chainloads the ISO's boot loader instead of booting the OS directly.

For reading the docs, I gather that.

1. Ventoy stores the location (disk, partition, file path) of the ISO as Ventoy Information Format.
2. The Ventoy Information Format gets saved in /dev/mem 0x80000~0xA0000 or /sys/firmware/efi/efivars/VentoyOsParam-77772020-2e77-6576-6e74-6f792e6e6574
3. The ISO is mapped with dmsetup from some magic initramfs.
4. The OS that boots sees the ISO as a regular device-mapper device and proceeds booting as usual.

In a "Ventoy Compatible" ISO, the 3rd step is performed by the OS that gets booted.

To make Archiso "Ventoy Compatible", we need to:

• Read and parse the "Ventoy Information Format" in initramfs (for the efivar, the first 4 bytes need to be stripped). Ventoy uses vtoydump, but maybe we can manage with ash and the utilities we currently have in the initramfs.
• Map or mount the ISO in initramfs.
• Mark the ISO as Ventoy Compatible by placing a ventoy.dat file in the ISO (probably is both the ISO and FAT file systems).