By default, when you don't have any network or when you didn't enable
a network mirror, your sources.list is empty. We don't want this
so we hook into the finish-install step to make sure to set it up
if it's empty.
In theory, this change is not really needed here because the
package is not installed by d-i, it's already installed by live-build
in the live image. But I want this file to be consistent with the
version in debian-installer...
* Add new --variant option to build alternative images of Kali.
* Add new --version option to decide of the name of the generated file.
* Add new --subdir option to indicate where the image must be moved.