initrd: Include hostname in udhcpc DHCP options

[eduarrrd]

Use case: host discovery for remote unlock. E.g.

• fixed address assigned based on the DHCP client's hostname (as opposed to e.g. MAC address)
• automated DNS record creation/update based on the DHCP client's hostname

Tested using Tcpdump.

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/)
• 23.05 Release Notes (or backporting 22.11 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.

[SuperSandro2000]

Suggested change

	udhcpcHostnameArg = if cfg.udhcpc.hostName != ""
	then "-x hostname:${config.networking.hostName}"
	else "";
	udhcpcHostnameArg = lib.optionalString (cfg.udhcpc.hostName != "") "-x hostname:${config.networking.hostName}";

[SuperSandro2000]

Also this should probably be rather a default or apply option.

[eduarrrd]

I've applied your suggestion, it's certainly cleaner.

Also this should probably be rather a default

What do you mean here? The value of the option has a sensible default already, no? Are you talking about some function I should be using that simplifies this?

or apply option.

I'm not sure what you mean by this. Some searching reveals a apply argument, but it doesn't seem official as per https://nixos.org/manual/nixos/unstable/index.html#sec-option-declarations

Does using it mean that value of cfg.udhcpc.hostName will be the string -x hostname:... ? If so, that makes re-use hard, no?