cmd: mtdparts: remove mandatory 'mtdparts=' prefix

cmd: mtdparts: remove mandatory 'mtdparts=' prefix

All U-Boot users must define the mtdparts environment variable with:
setenv mtdparts mtdparts=...

While this may ease the partition declaration job to be passed to
Linux, this is a pure software limitation and forcing this prefix is a
complete non-sense. Let the user to declare manually the mtdparts
variable without the prefix.

Signed-off-by: Miquel Raynal <[email protected]>
Acked-by: Jagan Teki <[email protected]>
Reviewed-by: Stefan Roese <[email protected]>