boot.initrd.systemd: make TPM2 modules optional

[elohmeier]

Description of changes

This is a follow-up to a #189676 (comment) reg. #189676. On some kernel configs the TPM2 modules are not available, e.g. the Raspberry Pi kernels. This change implements a proposed fix to only pull in the TPM2 modules if they are available, fixing the "module not found" issues for these kernels and re-enabling the use of boot.initrd.systemd.

Recreated #207969 to unstuck merging, see #207969 (comment).

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/)
• 23.11 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.