cmd: ubi: clean the partition handling

UBI should not mess with MTD partitions, now that the partitions are
handled in a clean way, clean the ubi command and avoid using this
uneeded extra-glue to reference the devices.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index c9be859..b571e84 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1840,6 +1840,8 @@
 	  capabilities. Please, consult the MTD web site for more details
 	  (www.linux-mtd.infradead.org). Activate this option if you want
 	  to use U-Boot UBI commands.
+	  It is also strongly encouraged to also enable CONFIG_MTD to get full
+	  partition support.
 
 config CMD_UBIFS
 	tristate "Enable UBIFS - Unsorted block images filesystem commands"