fng-install:u-boot:mx8mp: Adapt start of bootloader based on soc_id and partition

The mx8m plus soc expects the bootloader to start at offset 0 on the mmcblk0boot0 partition but on offset 32 at the mmcblk0 partition. Default was an offset of 33 for processors supported until now. This patch selects the correct offset.

Also fixes the selection of the boot partition accoring to partition scheme. The installation on mmcblk0boot0 didn't worked correctly before.