Don't try to guess the location of the NixOS config file

The NixOS config need not be $NIXOS_CONFIG, it can also be set through
-I nixos-config=... or not exist in a separate file at all (e.g. in a
NixOps deployment).

Issue #212.
Fix spelling

Also, it's not necessary to order a unit after "sysinit.target" since
that's implied.
Build 32-bit VirtualBox image

Issue #200.