initrd-ssh: allow empty hostKeys

[phaer]

This allows users to optionally allow empty host keys for initrd sshd. The original assertion seems to originate in the switch from dropbear to openssh about 3 years ago and makes it impossible to use sshd in your initrd without providing keys at build time. This would be useful for people like me who'd like to optionally generate one or provide them via oob mechanisms.

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• 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
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.11 Release Notes (or backporting 22.05 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
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• 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/3032/1309

[nixos-discourse]

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

https://discourse.nixos.org/t/prs-already-reviewed/2617/672

[ck3d]

LGTM

[RaitoBezarius]

I would in favor of:

• an option ignoreEmptyHostKeys
• keep the assert and use that flag to override it
• no warning if ignoreEmptyHostKeys is explicitly set

But I have no strong opinion on this.

[phaer]

@RaitoBezarius Thanks for your input! I agree that's a better design and just changed this PR to implement your recommendation instead.