ubi-utils: Add ubiblock tool

ubi-utils: Add ubiblock tool

With the addition of block device access to UBI volumes, we now
add a simple userspace tool to access the new ioctls.

Usage of this tool is as simple as it gets:

  $ ubiblock --create /dev/ubi0_0

will create a new block device /dev/ubiblock0_0, and

  $ ubiblock --remove /dev/ubi0_0

will remove the device.

Artem: slightly changed the header comment.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>