image: don't modify file permissions before rootfs generation

Modifying the file permissions can be harmful, as it would make files
world-readable even if they weren't in the ipk packages. The
Image/mkfs/prepare step is removed completely, as it is redundant now (/tmp
and /overlay are already provided by base-files with the correct
permissions).

It has been verified that this change does not affect any permissions of
files in the default package set except /etc/ppp/chap-secrets, which was
world-readable before. All packages not in the default set are more likely
to be installed via opkg than being part of a base image and thus were
usually not affected by the permission modification anyways.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>