/img/tmpfs is the hierarchy that will be copied to /mnt/tmpfs
during stage 1 init, so the basics are covered for stage 2:
 - /dev points to a real, writable device directory
 - /tmp points to a real, writable temporary directory
 - /service points to a real, writable scan directory that
already contains basic services (s6-svscan's own logger and
an early getty).