Adding support for extra entries and files.
The goal is to mimic the current nixos behavior, so our installer can act as a drop-in replacement. This mean:
- taking the extra files specified in the boot.json and placing them under $tmp/EFI/nixos/.extra-files
- taking the extra entries specified in the boot.json and placing them under $tmp/loader/entries Application of the modification is not part of this issue directly, but checking for breaking change should be done.