Enable graphics for imx6ul/ull boards. Add panel and backlight drivers

Set environment variables for impoved menuconfig support

Use generic names for the boot scripts for booting from a single
partition and for doing A/B boots from two separate partitions.

Use the generic 'load' command instead of specific filesystem
commands.

Also sync the environment for i.MX6UL/ULL with the one for i.MX8MM.

This way we can boot i.MX8M devices with a boot logo and splash
screen as the display pipeline is available before the userspace and
filesystem are ready.

In the future we will reference the BSP as meta-ked-bsp. Adjust names
of layers, files, etc. accordingly. Only some OE metadata like image,
distro or machine override names are keeping the 'ktn' for now.

KED stands for Kontron Electronics Deutschland GmbH where the BSP
is maintained. Nowadays it support only products of the KED branch
and products of the Kontron Europe GmbH or other branches are not
supported.