This feature adds same systemd-generator that distrobuilder adds too.
As long it's all distrobuilder does too, we can generate fully
functional lxc images on m-p side.

This further refines the modular build by making
metadata being a clearly separated feature rather
than having to rely on runtime tests, and also by
moving the code which cares for kernel bits of base
installation (.base list) in a feature of its own.

There's more to it but let's get the ball rolling first.