Mounting of images without an hierarchical partition structure and a defined / implemented.

If no mount for / (root) at given mount point can be established before mounting a sub partition, a virtual mountpoint
with a small tmpfs (ramdisk) is set up for / (root), so also non hierarchical images without a defined root partition
can be set up. This is usable, if partitions of an image are not mountable at one hierarchical mount tree.
Unmounting the image automatically removes the ram disk.

Signed-off-by: Thomas Mittelstaedt <thomas.mittelstaedt@de.bosch.com>