`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).
50648f568d