`nixos/modules/installer/kexec/kexec-boot.nix` doesn't contain any
custom NixOS config, other than importing `netboot-minimal.nix` (which
imports `netboot-base.nix`, which imports `netboot.nix`.
`netboot.nix` really is just describing a self-contained system config,
running entirely off kernel and initrd, so we might as well move the
kexec script generation there as well.
`netboot.nix` already contains some `system.build` attributes.
Provide a `system.build.kexecTree` attribute (and `kexecScript` for
composability).
You can now run a test in the nixos/tests directory directly using
nix-build, e.g.
$ nix-build '<nixos/tests/login.nix>' -A test
This gets rid of having to add the test to nixos/tests/default.nix.
(Of course, you still need to add it to nixos/release.nix if you want
Hydra to run the test.)
