base-files: Temporarily support filesystems directly on MTD
Due to a buggy flash driver on the Econet platform, we would like to temporarily avoid writing to flash as much as possible. Temporarily add support for root filesystems stored directly on MTD to iopsys-system-layout.
This support can be removed, once the issue is resolved, because we do not want to support non-UBI setups in the long term. The half-way working setup, that this enables, is to have the rootfs on mtd and the overlay on tmpfs.