Build host tools on non-FreeBSD host with DIRDEPS_BUILD

Add Makefile.depend.options to libnetbsd, mkimg and makefs
to ensure libegacy is build if needed.

targets/pseudo/stage/Makefile avoid the need for mtree
the staging process creates target dirs as needed anyway.

Reviewed by: stevek
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D39757