This change adds HOST_VARIANT_DIR so the various BSD OS dependent
code can be separated into its own directories rather than
using #ifdef's.

This may also allow an BSD variant OS to host another BSD variant's
executable as a target.

Signed-off-by: Sean Bruno <sbruno@freebsd.org>
Message-id: 1402246651-71099-2-git-send-email-sbruno@freebsd.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>