RISC-V - SBI on Litex FPGA SoCs and other hardcores

The SBI interface on linux-on-litex-vexriscv allows us to boot Linux, on a 32-bit vexriscv RISC-V processor, with a few tiny peripherals attached.

This should allow us to run Linux on the smallest 25k LUT FPGAs from Xilinx, or Lattice Semiconductor amongst others.