for building parts of the system. E.g.
$ nix-build /etc/nixos/nixos -A upstartJobs.xserver
to build the Upstart job for the X server, or
$ nix-build /etc/nixos/nixos -A kernel
to build the NixOS kernel.
* /etc/profile.sh: if ~/.nix-defexpr doesn't exist yet, create it as a
directory and add links to root's channels, /etc/nixos/nixos and
/etc/nixos/install-source.nix (as "nixpkgs_sys").
* boot.useKernel -> boot.kernel.
svn path=/nixos/trunk/; revision=9334
