nixos-init: read required data from boot.json

[r-vdp]

This PR does two things:

1. pass info to nixos-init through bootspec. This has several advantages, it makes it easier to run nixos-init since you only need to supply the toplevel path, which also makes it easier to re-use it in the future to replace the activation script, and it also allows us to clean up some files in the toplevel (like the etc overlay stuff) and instead pass all that info through bootspec.
2. make it possible to pass / as the prefix. This is a first step to be able to re-use nixos-init during reactivation.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[r-vdp]

Somehow the etc-overlay is failing after the latest changes, I'm looking into it

Fixed, we use nixos-init for the find-etc service even if nixos-init isn't explicitly activated, so we need the extra bootspec extension independently of whether nixos-init is enabled.

[nikstur]

we use nixos-init for the find-etc service even if nixos-init isn't explicitly activated, so we need the extra bootspec extension independently of whether nixos-init is enabled.

This effectively makes the systemd initrd depend on bootspec now. Since the systemd-boot builder already depends on bootspec, however, this is fine IMO.