osstest: add support for the FreeBSD package manager

osstest: add support for the FreeBSD package manager

FreeBSD support is added to target_install_packages and
target_install_packages_norec, although there's no equivalent to the
--no-install-recommends in the FreeBSD package manager.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
---
v19: Rework on top of target_run_pkgmanager_install

Changes since v6:
 - Use $ho->{Os} in order to detect the target OS.

Changes since v5:
 - Add prototype for package_install_cmd.
 - Use $ho->{Ident} instead of hardcoding "host" in get_hostflags.

Changes since v4:
 - Unify more code between FreeBSD and Debian: introduce an
   OS-agnostic package_install_cmd to generate the install cmd.

Changes since v3:
 - New in this version.