Draft: Add bootable APT image for imx8 SoC

This an initial (draft) support for Variscite VAR-SOM-MX8M-NANO board. See phab task https://phabricator.apertis.org/T7881

Related MRs on dependencies:

• pkg/arm-trusted-firmware!5 (merged)
• pkg/firmware-nonfree!13 (closed)
• pkg/u-boot!36 (merged)