When using the apt packagemanager kiwi required the use of
debootstrap to create the initial rootfs. This works as long
as there is always a main distribution repository available
which follows the structure of the official debian mirrors.
However if such a main distribution is not present or an
alternative layout like e.g OBS repos is used, debootstrap
will refuse to work. To allow for an alternative and without
the dependency to debootstrap kiwi supports using a prebuilt
bootstrap package providing the mini rootfs to serve as
the bootstrap result. As all other package managers properly
supports installation into an empty new root, this feature
was only added when using the apt packagemanager
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