Handle config.system.build type change from lazy attrset to submodule

[tomeon]

Cherry-picked from #155.

Don't use lib.mkForce on the system.build.kexec_tarball definition when building on a nixpkgs version from before the commit that changed config.system.build's type from a lazy attribute set to a submodule.

Prior to this type change, there is no system.build.kexec_tarball option declared, so the NixOS module system does not resolve priorities/overrides in the config.build.kexec_tarball definition.

That is, with lib.mkForce, the config.build.kexec_tarball definition ends up being something like:

{
  _type = "override";
  content = <...>;
  priority = 50;
}

Removing lib.mkForce permits sensibly interpolating system.build.kexec_tarball in system.build.kexec_bundle's builder script.

Likewise, no lib.mkOverride for system.build.raw.

This PR introduces two shim functions defined in ./lib.nix, maybe.mkForce and maybe.mkOverride, that try to detect whether config.system.build is a submodule and:

1. if so, pass through to lib.mkForce or lib.mkOverride, and
2. If not, just return the supplied argument as-is.