release: Add an image for CI

A lot of projects CI can't do FreeBSD tests currently.
The main reason is that the project CI infrastructure is runned on Linux
and that our images aren't modifiable from a Linux hosts.
Add a basic image specific for this case (called BASIC-CI for a lack of a
better name).
The image have no package pre-installed.
It only have a few modification to have dhcp client runned on the default
interface and sshd started with option to be able to log on without a password
as root.

Sponsored by: The FreeBSD Foundation

Reviewed by: re (gjb@)
Differential Revision: https://reviews.freebsd.org/D25598