Generalize Bootspec beyond NixOS toplevels

[colemickens]

Hi,

I've been putting off creating my own installer for a while, not that that is particularly interesting on its own. But, it would be really neat to have a "Ultimate Custom NixOS Installer" that has:

• my "installer" config for both aarch64 + x86_64 (systemd-boot will filter if the entries have their arch declared)
• (ideally the new OSS fork of) memtest enabled

However, it looks like the bootspec/generator parts are very specific to NixOS generations, and maybe even specifically on the generations existed as "system" profiles. Is this a strict part of bootspec's design?

Instead of the current nixos module which seems to just always invoke the generator+installer in a single, automatic way, there would be an available API to say here's "the exact toplevels+extra-bootables that I want turned into a bootloader package". I think this would also go most of the way to making a cleaner method of creating images/installers, since you don't need to interactively boot the VM to do the final bootloader installation step.