[RFE] Add RISC-V support for Flatcar Container Linux

[Adrian Vladu]

Current situation

Flatcar cannot be built for the RISC-V architecture (https://riscv.org).

Impact

Consumer boards are starting to become widely available, like the Vision 5, LicheePi 4A or Milk-V. Enterprise grade manufacturers like Ventana are also providing server grade RISC-V CPUs.
Also, cloud providers like Scaleway already offer RISC-V instances.

Ideal future situation

To be able to run Flatcar on RISC-V systems.

Implementation options

Upstream Gentoo already has support for most of the packages. The SDK needs to be worked on and maybe also streamline / organize the build code for ARM64 and RISC-V at the same time, so that ARM-on-ARM and RISC-on-RISC builds can be possible, without forcing cross-compilation.