osstest: change the meaning of need_build_host

osstest: change the meaning of need_build_host

Make need_build_host store a string instead of a boolean. This is
later going to be expanded to handle the FreeBSD build jobs.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
---
Changes since v5:
 - Use a regsub to set need_build_host.
 - NB: I would prefer to keep the tolower instead of changing the
   procs to -FREEBSD/-LINUX since I don't really like the SHOUTING.

Changes since v4:
 - Fix the usage of the catching-otherwise blocks.

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