Draft: seco-mx6: prepare for edgehog

Hello Jonas.

I`ve created this MR, so you can also see changes for u-boot. Similar to seco-ne/yocto/layers/meta-seconorth-machine!406 (closed)

In short:

• changed mmc order to match linux and edgehog u-boot
• switched to take env from /dev/mmcblk0boot0 with 1.5 MB offset.
• added board code to set "fdt_file" variable with autodetected base dtb
• copied some variables from edgehog env (should be ported fully and properly most likely) to make edgehog boot script work