This is for instance useful on Asahi where an additional initrd archive
containing firmware blobs and per-device calibration files is placed
on the ESP and updated by the Asahi Linux Installer.
These need to be loaded alongside the NixOS initrd.
Implemented for systemd-boot and Limine. Grub is left out since its
install script does not use bootspec yet.
Co-authored-by: Florian Klink <flokli@flokli.de>
