colibri_imx7: split and resize firmware MTD partition

Use two separate partitions for the two firmware instances. Also
resize them to be of the same size which also makes the start of
the UBI partition nicely aligned to 0x400000.

In order to detect the new MTD layout and whether we run a U-Boot
with the new BCB format or not, introduce a variable called
"updlevel" which we can use in update/upgrade scripts.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Acked-by: Max Krummenacher <max.krummenacher@toradex.com>