cmd: ubi: Add 'ubi list' command for printing list of all UBI volumes

To allow easily iterate over all UBI volumes, add a new command which
either print all user UBI volumes on output or set them into env variable.

As UBI volumes can have arbitrary name/label, in most cases it is useful to
iterate them by their numbers. This can be achieved by -numeric flag.

This functionality is similar to already existing 'part list' command which
prints partitions on formatted block device.

Signed-off-by: Pali Rohár <pali@kernel.org>