If no mount for / (root) at given mount point can be established before mounting a sub partition, a virtual mountpoint
with small tmpfs (ramdisk) is set up for / (root), so also non hierarchical images without a root partition can be set up.
This is usable, if partitions of an image are not related to each other. Unmounting the image
automatically removes the ram disk.
Some error corrections like logging function instead of echo.
Some sleep 1 is needed after partprobe, because other tools to get labels and other partitions sometimes
work on useless info of device.