mtd: Add ubiblock tool

The new release adds a new userspace tool called ubiblock. This is
used to control UBI's new block device feature which will be added
in Linux v3.15.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>