Deprecate and remove stage-0 as a Mobile NixOS implementation detail

[samueldr]

Part of #686

---

The deprecation will start with:

• Deprecation notice/warning on systems with stage-1 enabled.
• Deprecation notice/warning on systems booted from stage-0 on activation [nixos-rebuild switch/boot] (using the DT nodes as a hint).
• Removal of supportsStage-0 from devices enable

Then we'll be able to, at some point later, remove it entirely.

In the end, #291 would happen, or the moral equivalent of it. An external project (or many different ones) could act just like any other bootloader within NixOS semantics.

Though I'm not promising that I will create and maintain any project like that in any timeframe. (kexec on ARM systems has loads of YMMV-ness to it...)

---

Relevant refs:

• PinePhone functionality #660 (comment)
• Disable warning about not having a bootloader #682 (comment)
• stage-0: Add ability to boot in generation kernel+initrd #262 (comment)
• pine64-pinephonepro: Init #535 (comment)
• pine64-pinephonepro: Init #535 (comment)