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
514fb24a45