configs/mx6cubox: bump U-Boot version

Since 2020.04 version, the enumeration of MMC has changed, so the boot
script must be updated

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>