Use ln -sf otherwise nanobsd.sh -wkWK fails
since the etc link exists already.

Before creating the nanobsd image nanobsd.sh creates a soft link for
usr/local/etc in the function setup_nanobsd()

This fails if the link already exists (when using nanobsd.sh -wkWK)

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D49873