Some changes for mountimage tool

- 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.

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