MFC r330249-r330250

r330249:
stand: Makefile SUBDIR cleanup

Use SUBDIR.${MK_*} where appropriate. r330248 eliminated most of the
offenders, sweep the rest under the rug.

Differential Revision: https://reviews.freebsd.org/D14545

r330250:
stand: Fix build after r330249

One does not simply convert to SUBDIR.yes in stand without making everything
else in the affected files SUBDIR.yes -- there are better ways to do this.