base-files: system_layout: Enable get_root_device_type wo args

Enable calling get_root_device_type without any argument. Previously it required the rootfs-device as first argument. Now we support both calling it with the root-device and without. When called without, the function will get the root-device on its own. Refactor the rest of the code to enable this functionality.

The reason, why we support both is that in some situations, you need both the root-device and the type and their is no need to retrieve the root-device twice.