[Toybox] Hosted FreeBSD CI for toybox

[Ed Maste]

I've been looking at Cirrus-CI as a hosted CI platform targeting
FreeBSD and added an experimental config to build & test toybox on
FreeBSD.

The config file (.cirrus.yml) is just:
-----
freebsd12_task:
        freebsd_instance:
                image: freebsd-12-0-release-amd64
        install_script: pkg install -y bash gmake
        script:
                - gmake allnoconfig KCONFIG_ALLCONFIG=freebsd_miniconfig
                - gmake
        check_script: gmake tests
-----

And an example result can be found at
https://cirrus-ci.com/task/6202688419135488

Cirrus-CI supports Linux, Windows, macOS and FreeBSD - build & test
config for the other platforms can be added to the same config file.

Is this something desirable for the upstream repo?