imx: imx8mm-beacon: Move Environment to eMMC partition 2

The downstream U-Boot distributed by Beacon stores the environment
in the eMMC and the end of partition 2.  This allow the environment
to stay on the SOM regardless of the boot source.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>