osstest: add support for FreeBSD buildjobs to sg-run-job

osstest: add support for FreeBSD buildjobs to sg-run-job

Add support and introduce a FreeBSD build job to sg-run-job.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
---
Changes since v5:
 - Add a '+' to the arguments passed to ts-freebsd-set-hostflags, so
   they are hidden from testid.

Changes since v4:
 - Use a switch in allocate-build-host.

Changes since v3:
 - New in this version (split from existing patch).