Need to use ${${_${group}DIR_${file}}} for STAGE_DIR

STAGE_DIR.${${_${group}DIR_${file}}:C,[/*],_,g} was getting
${STAGE_OBJTOP}BINDIR rather than
${STAGE_OBJTOP}${BINDIR} when FILESDIR=BINDIR

Reviewed by: stevek
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D21858