The Khadas VIM (http://khadas.com/vim/) is an arm64 DIY Set-Top-Box based on Amlogic P212 reference board that use S905X SoC.
As Odroid-C2 (based on S905 SoC) is in the mainline U-boot, it should be possible to adapt it for the Khadas VIM (of course a lot of work are needed!).