etc: Don't create stray directories in NO_ROOT distrib-dirs

The loop above is responsible for creating the actual directories,
whilst this one is just responsible for creating the corresponding
METALOG. Since DESTDIR already includes DISTBASE, this results in
creating a second set of roots (one per MTREES entry) within DISTBASE
whenever DISTBASE is non-empty, such as base/base, base/base/var,
base/base/usr, etc. in the distributeworld case. This is purely cosmetic
though as they won't appear in the METALOG.