this lets us *dis*able filesystem explicitly, as is required by e.g. the
zfs-less installer images. currently that specifically is only easily
possible by adding an overlay that stubs out `zfs`, with the obvious
side-effect of also removing tooling that could run without the kernel
module loaded.
This includes disabling some features in the initrd by default, this is
only done when the new initrd is used. Namely, ext and bcache are
disabled by default. bcache gets an own enable option while ext is
detected like any other filesystem.
