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