[PoC] Add RISC-V support

[ader1990]

Add initial support for RISC-V. See: https://github.com/flatcar/Flatcar/issues/1420

What works:

• SDK build
• Root filesystem build

What does not work / to be done:

• a PoC with a hardware board running Flatcar
• image_to_vm.sh -> more investigation is required on how to make the grub2 bootloader / partitions work with the dm-verity on RISC-V
• secureboot - needs to be short-circuited
• The kernel - current hardware boards have their own bootloader / kernel partitions that are custom-made and Flatcar's A/B partition scheme and upgrade workflow cannot work as such
• Upstream the required bits to Gentoo