osstest: introduce a FreeBSD build script

osstest: introduce a FreeBSD build script

In order to generate the FreeBSD installer image and the install
media.

The install sets are the vanilla ones generated by the 'ftp' release
target.  The installer image is handcrafted based on the filesystem
created by the 'bootonly' target, which is then populated with the ssh
host keys, and setup in order to use the serial console. The other
difference from upstream FreeBSD installer images is that the one
built by osstest uses a ramdisk instead of relying on the installer
media to be somehow attached, either on a CD or USB drive. This is
required in order to boot the image from pxelinux (where no CD or USB
is actually attached to the host, and everything is fetched from
tftp).

Due to the nature of the FreeBSD build, the outputs are different from
what osstest expects from a buildjob, more specifically
path_freebsddist points to a folder that contains the several outputs
form this buildjob.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
---
Changes since v4:
 - Remove some useless comments.
 - Use qw(git) instead of 'git'.
 - Replace unicode nonbreaking space.
 - Remove trail ','.
 - Improve the way to fetch the version number.

Changes since v3:
 - Introduce two helpers to perform the build process.
 - Install packages using target_install_packages.

Changes since v2:
 - Increase build target timeouts.
 - Use sysrc instead echo to set rc.conf options.

Changes since v1:
 - Remove the ts-build-check FreeBSD hack.
 - Use pkg-static instead of pkg.
 - Introduce buildcmd_stamped_logged_root and target_cmd_build_root.
 - Use target_cmd_build_root and buildcmd_stamped_logged_root in the
   ts-freebsd-build script.
 - Fix the script snippets to use <<END.<<'END' in order to avoid
   escaping the shell variables.
 - Set path_freebsddist runvar to point to the folder where the build
   files are stashed.
 - Add a comment at the top of the file describing what runvars are
   consumed/produced by the build script.