Add new consolidated mkimage scripts

These new scripts are streamlined such that, for example, "contrib/mkimage/debootstrap" is _only_ responsible for filling a directory with the results of running debootstrap, and it can accept any arbitrary arguments.  This gives us a nice clean separation of concerns, makes these scripts trivially easy to write, and allows them to accept any arbitrary arguments (like "before-post-install" as a rinse argument, for example).

Fixes #2442
Fixes #4496
Closes #4807