This code was using "-efi-boot-part", which works fine for booting but
creates images that d-i can't use: only the full disk or the ESP
partition would contain working filesystems, and d-i doesn't like that.
Instead, switch to appending the ESP as an extra partition on the end
of the image. This will take a small amount of extra space, but will
at least work reliably!
Remove un-needed boot$N in the xorriso command line, we don't have
crappy BIOS lomitations.
Remove support for older xorriso versions.
Use new -efi-boot-part --efi-boot-image options to make an
isohybrid-style images with an explicit EFI partition. Will hopefully
make d-i happier with our images from USB.
Steve McIntyre