clean_build is not always done (*), so move the mkdir to the first step
that needs it and is always done: make.conf build.

(*) In our build environment we actually never do a clean build unless
instructed to do so, to make sure we do not accidentally delete the
built world when on a tight schedule.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275180 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f