[OpenNebula-images] Add scripts to generate FreeBSD images.
Tested for 12.1-RELEASE, 11.3-STABLE should be tested as well.
Two scripts are added, FreeBSD-build-opennebula-image-generic.sh takes information from the environment and generates the specified image with either dualstack or IPv6-only kernel and base applications.
FreeBSD-build-opennebula-image.sh has the versions supported in DCL and generates all neeeded images (combination of Version x Network support).
This should run in a DCL VM with an OK amount of cores (4/8 minimum), 2-4G RAM, and storage of roughly 20G + 5G * #resulting_images.
This is because there is the base system, a 'pristine chroot', and during the build there can be 2 copies of the resulting system written to the system. Since there are 4 combinations of images: {STABLE,RELEASE} x {dualstack, IPv6ONLY}
That means we'll need to assign about 40G storage to be on the safe side.