Create boot part via WIC and add option to enable eMMC boot flow

On i.MX8M we have devices with SPI NOR and eMMC. Historically we used
the SPI NOR as primary boot device to store the bootloader and the
environment.

This adds an option to select eMMC boot to store the bootloader and
environment in the hardware boot partition of the eMMC.

Furthermore we try to avoid custom code to generate the boot
partition and use WIC and a custom swupdate script instead.

To make sure the correct fw_env.config is installed on the system,
generate it dynamically depending on the selected options and
platform.