…ghtly by integrating bootspec

Relocate to $out/boot.json, top-level key of specification version

…der backends (NixOS#125)

* RFC 0125: Use an intermediary representation (bootspec) for bootloader backends

Bootspec is a set of memoized facts about a system's closure.
These facts are used as the primary input for bootloader backends like
systemd-boot and grub, for creating files in `/boot/loader/entries/`
and `grub.cfg`.

In this proposal we create a stable, comprehensive, and machine-parsable
definition of a NixOS Generation as an intermediate representation (IR)
between the NixOS system definition and the bootloader management tools.

* bootspec: relocate to $out/bootspec/boot.v1.json

* Clarify that this document describes bootspec v1

* Note that the filename must match the contained schemaVersion.

* Replace the specialisation's pointer to a bootspec with the full bootspec document

* Clarify the 'to interface with' bit

* Link to the bootspec implementation PR

* Note it is likely for bootloader backends to change their results slightly by integrating bootspec

* Relocate to $out/boot.json, top-level key of specification version

* Fix nits from code review

Co-authored-by: Cole Helbling <cole.e.helbling@outlook.com>
Co-authored-by: Linus Heckemann <git@sphalerite.org>

* 0125-bootspec: add the shepherd team and leader

Co-authored-by: Kevin Cox <kevincox@kevincox.ca>

* 0125-bootspec: initrd is optional, add system, extensions, clarify the meaning of optional

* PR: address comments (1)

Co-authored-by: Cole Helbling <cole.e.helbling@outlook.com>

* PR: address comments (2)

Co-authored-by: Cole Helbling <cole.e.helbling@outlook.com>

* Update RFC text with changes agreed on with RaitoBezarius

---------

Co-authored-by: Cole Helbling <cole.helbling@determinate.systems>
Co-authored-by: Cole Helbling <cole.e.helbling@outlook.com>
Co-authored-by: Linus Heckemann <git@sphalerite.org>
Co-authored-by: Kevin Cox <kevincox@kevincox.ca>
Co-authored-by: Raito Bezarius <masterancpp@gmail.com>