osstest: introduce a script to create a FreeBSD flight

osstest: introduce a script to create a FreeBSD flight

The logic to create a FreeBSD build job is added to
make-freebsd-flight. This includes creating a FreeBSD build job, and
also testing the output of that build job (by creating another build
job that depends on the output of the first).

Note that the FreeBSD build job needs some input in order to setup a
FreeBSD host, and that can be fetched from different places:

1. Env variable FREEBSD_<arch>_BUILDJOB: use the output from a
previous build-<arch>-freebsd job.

2. Env variables FREEBSD_DIST and FREEBSD_VERSION: set before calling
into make-flight, provide the path to the installer image and sets
folder and the version being installed.

3. Config file FreeBSDDist and FreeBSDVersion: same as 2. except that
they are set on the config file.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
---
Changes since v6:
 - Re-wrap lines.
 - Remove the freebsd hostflag.
 - Do not set the FreeBSD git trees in this patch.
 - Set all_host_os runvar to freebsd for the FreeBSD build jobs.

Changes since v5:
 - Append the "freebsd" hostflag, the specific version will be set
   dynamically by the ts-freebsd-set-hostflags script.
 - Use FREEBSD_<arch>_BUILDJOB to set the freebsdbuildjob runvar.
 - Loop around arches, although only amd64 is supported ATM. This
   should make it easier to add more arches later on.
 - Expand README comment to describe the expected layout of the
   FreeBSD install media.

Changes since v4:
 - Convert get_freebsdjob_runvars into set_freebsd_runvars, mimic the
   behavior of set_hostos_runvars.

Changes since v3:
 - Split the sg-run-job code into pre-patches.

Changes since v1:
 - Replace freebsd_buildjob with freebsdbuildjob.
 - Replace FREEBSD_SETS/IMAGE with a single FREEBSD_DIST that points
   to a folder that should contain both things.
 - Document the FreeBSDDist and FreeBSDVersion config file options in
   the README file.