Some instances are provided with a “local boot” option. As far as I understood from the rather vague documentation, local boot uses a EFI partition.
However EFI variables are not accessible in rescue mode, so a “normal” custom installation is not really possible.
So the question is: what’s the way to actually create a real custom image from scratch? Or equivalently, how can I set up my virtual instance to boot my initramfs/kernel from my EFI partition?