imx: ventana: set dynamic env var for flash layout

imx: ventana: set dynamic env var for flash layout

NAND devices have differing layouts with respect to page size and pages per
block. These parameters affect the parameters that need to be passed to
mkfs.ubifs and ubinize used to create UBI images. The various NAND chips
supported by Gateworks Ventana fall into two different layouts which we
refer to as 'normal' and 'large'. This layout is useful when referencing
ubi files to download and flash so we create a dynamic env variable for it.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>