nixos/iso-image: improved grub and media config

[gray-heron]

Description of changes

Added capability to add custom grub options and modify contents of the Boot Catalog of the iso images produced by the iso-image installer module. This, for example, allows users to create custom, pre-installation, pre-boot routines, such as hardware tests, BIOS updates, etc.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Extensively tested added functionality
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-ready-for-review/3032/3764

[misuzu]

Please also rebase your PR to make sure the new CI passes

[misuzu]

I had another look and I don't think I like it. The isoImage.bootContents seems redundant given there's already isoImage.contents and this change also doesn't handle the syslinux menu.
I think something like this but which also handles the syslinux part would be a right way and would also allow replacing all entries and not just adding extra stuff. It won't be easy though...

[gray-heron]

When you say redundant do you mean it's possible to write to the El Torito boot catalog with just isoImage.contents? How?

Okay, I will look into the bootloader entries, it shouldn't be that difficult to make something like grubEntries and syslinuxEntries. Just a question what to expose so it makes most sense. Any ideas appreciated.

[misuzu]

When you say redundant do you mean it's possible to write to the El Torito boot catalog with just isoImage.contents? How?

It's already handled by https://github.com/NixOS/nixpkgs/blob/3ee96dbd19fe1487a6c987b88963081c699e7309/nixos/lib/make-iso9660-image.sh and it's unrelated to the grub/syslinux menu entries so I'm no sure what you mean.

Just a question what to expose so it makes most sense. Any ideas appreciated.

I think it makes sense to be able to change the names of menu entries and kernel command lines without having to deal with the kernel/initrd details needlessly.