Clean up all directories created by make hier

The logic I introduced in r322511 unfortunately left chflags schg'ed
directories behind created by make hier (in the stock /etc/mtree
files, this is limited to /var/empty).

The proposed change calls chflags -R 0 and rm -Rf ... to clean all
of the directories that could not be removed by ${MAKE} clean.
${MAKE} clean in bsd.obj.mk calls cleandir/cleanobj, which handles
the first directory tree walk/removal.

Approved by: emaste (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18641