Firecracker: Make root disk configurable

Add a FCROOTFSSZ variable which defaults to "1g" which controls the
size of the Firecracker root filesystem; it can be set as low as "300m"
at present.

Allow WITHOUTS to be overridden if users want to build a root disk with
more -- or fewer -- parts of the FreeBSD base system.

Reviewed by: gjb
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D41041

release: Add code to make FreeBSD/Firecracker

This just stages the kernel and builds a stripped-down rootfs for use
with the Firecracker VMM. At some point in the future the release
engineering team might start publishing these, but initially it's
just here to simplify FreeBSD/Firecracker development and testing.

Note that the rootfs generated:
* Uses an IP address of 10.0.0.2 with a gateway of 10.0.0.1,
* Has sshd enabled,
* Has user "freebsd" with password "freebsd" and a root password
of "root", and
* Is 1 GB in size (but has growfs enabled).

All of those are subject to change without notice; anyone intending to
use FreeBSD/Firecracker in anything remotely resembling a production
environment should talk to cperciva first.

Reviewed by: gjb
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D40956