Code review comment for lp:~xnox/debian-cd/unbreak-grub2-hybrid-iso

This is definitely an improvement (as in, the ISOs boot now in all of {legacy,uefi}x{cdrom,disk}) so we should merge it.

This and recent changes have the side effect of making the ISOs bigger, because each of the four boot modes described above gets its own copy of grub. In older images, legacy-disk boots used the 432 byte (!) isohdpfx.bin to jump to the grub in the el torito boot image for legacy boot and the MBR EFI partition used by uefi-disk pointed at the same data as the el torito boot image for EFI. These tricks require creating overlapping partition tables though and the libisofs on focal, at least, doesn't let you do that (at least in my tests).

This is the best documentation I've found for all this btw: https://wiki.osdev.org/El-Torito.

I might propose a follow on commit that adds a *lot* of comments ...