bsdinstall avoid subdir depending on parent

When not doing tree walks, it is bad for sub-dirs to depend on
parents. Move the generation of opt_osname.h to distextract
and have others that need that depend on it.

In usr.sbin/bsdinstall use SUBDIR_DEPEND_ so tree walking still works.

Reviewed by: obrien
Differential Revision: https://reviews.freebsd.org/D39742