Fix building host tools for host

Several makefile depend on tools built for host.
At least when using DIRDEPS_BUILD we can build these for the
pseudo machine "host" to facilitate building on older host versions.

Ideally we would build these tools in their own directories to avoid
building more than needed.

For now, setting an appropriate default for BTOOLSPATH will suffice

Reviewed by: stevek
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D39708