non-update build breaks w/old DESTDIR

["John D. Baker" <jdbaker%mylinuxisp.com@localhost>]

If I start a non-update build (w/o "-u") and DESTDIR still exists from
previous builds, METALOG is not updated properly and the build fails
when populating the installation image (armv7.img) complaining (usually)
about files on filesystem shorter than size recorded in METALOG.

To workaround, wipe the DESTDIR when doing a non-update build.  Subsequent
update builds don't have a problem.

I've encountered this behavior only when building evbearmv7hf-el (the
only ARM arch I have right now).  Non-arm ports have no problems with
non-update builds with stale DESTDIR.

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