release: add optional OCI images

This adds three OCI archive format files to the release containing
FreeBSD base images suitable for static linked, dynamic linked and shell
workloads. The shell image also contains pkg-bootstrap and can be easily
extended by installing packages (including pkgbase packages).

Approved by: re (cperciva)
Reviewed by: dch, cpersiva, jlduran, zlei
Differential Revision: https://reviews.freebsd.org/D46759
MFC after: 2 days

(cherry picked from commit d03c82c28da86e0812b98b051d24ae5980804ad7)
(cherry picked from commit 6686056ca3547c8280bd2423c59c661c2f501409)