Expose debootstrap include/exclude packages in the manifest.

This is useful in many cases in which the next steps of bootstrapping the image depends on the packages, for e.g to install packages using non-conventional repository transports like `https`.

_Note_: I got confused while trying to add docs, I assume the docs are stored in `gh-pages` branch, so should I update the branch and give another PR - or is there any other workflow that should be followed?