cmd: ubi: add a command to rename volume

This commit adds the command ubi rename to rename an ubi volume.
The format of the command is: ubi rename <oldname> <newname>.
To enable this command, the option CMD_UBI_RENAME must be selected.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>