cirrus: Add Cirrus CI integration for FreeBSD build

cirrus: Add Cirrus CI integration for FreeBSD build

CirrusCI [1] is free for open-sorce projects and provides similar
to TravisCI interfaces. One significant difference is ability
to run tasks on FreeBSD instances.

This patch adds simple configuration file to test OVS build
on two FreeBSD releases with gcc and clang.
Unit tests are commented out because they are broken for now.

To enable the automated checks Cirrus CI application from GitHub
Marketplace should be installed. See details in Quick Start guide [2].

[1] https://cirrus-ci.org
[2] https://cirrus-ci.org/guide/quick-start/

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>