mkimage implementation requires that the variables
to be passed to the scripts are to be prefixed with
GLOBAL_ or INFO_ tags as appropriate; in this case
the upstream makefile didn't care to.

A virtual machine isn't very useful if there are no means
to access it; let's bring up the basic networking and provide
root SSH access via pre-existing public key.

As the remote access with known default credentials is roughly
equivalent to just lending one's VMs to anyone with network
access to it, the fallback root password is now exterminated;
you have to provide one (or a long enough random string
if you plan to use keys only, see e.g. apg utility).

It appears that reusing installer-feature-*-stage3 packages
is perfectly fine with VM images; these just need to be removed
after the package scripts they carry have worked out.