cmd: mtdparts: describe as legacy

The 'mtdparts' command is not needed anymore. While the environment
variable is still valid (and useful, along with the 'mtdids' one), the
command has been replaced by 'mtd' which is much more close to the MTD
stack and do not add its own specific glue.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>

cmd/Kconfig

@@ -1699,7 +1699,11 @@ config CMD_MTDPARTS
 	bool "MTD partition support"
 	select MTD_DEVICE if (CMD_NAND || NAND)
 	help
-	  MTD partition support
+	  MTD partitioning tool support.
+	  It is strongly encouraged to avoid using this command
+	  anymore along with 'sf', 'nand', 'onenand'. One can still
+	  declare the partitions in the mtdparts environment variable
+	  but better use the MTD stack and the 'mtd' command instead.
 
 config MTDIDS_DEFAULT
 	string "Default MTD IDs"