DISK: Add common partition handling for disk like media ...

This covers disk like media like SD card, CF cards, regular hard disks and also
USB mass storage devices. Most common used partition table is still of DOS type.
This implementation is prepared to support more partition types in the future.

Signed-off-by: Juergen Beisert <jbe@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>