This is to allow building an image for a 64bit machine but with 32bit userspace.
Probably not a common usecase but ideal if you need to address more higher quantities of memory but cant migrate to a full 64bit userspace due to something like ruby eating twice as much memory.
This is useful in many cases in which the next steps of bootstrapping the image depends on the packages, for e.g to use a non-conventional repository transports like https.
The tarball bootstrap method now uses an existing tarball if it is
found, regardless of its settings on the manifest, although it will
create a new one only if `"tarball"` is `true`.
This closes#24.
