mtd: uboot: search for an equivalent MTD name with the mtdids

Using an MTD device (resp. partition) name in mtdparts is simple and
straightforward. However, for a long time already, another name was
given in mtdparts to indicate a device (resp. partition) so the
"mtdids" environment variable was created to do the match.

Let's create a function that, from an MTD device (resp. partition)
name, search for the equivalent name in the "mtdparts" environment
variable thanks to the "mtdids" string.

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