Getting started with Buildroot Linux on RISC-V with SymbiFlow tutorial

# Tutorial for SymbiFlow with Buildroot Linux on RISC-V

# Brief explanation

Write a tutorial for getting started with SymbiFlow on a Linux-enabled RISC-V CPU on a mid-range FPGA with Buildroot Linux.

## Expected results

An easily reproducible tutorial which enables people to work with SymbiFlow on a more advanced and very useful example system using Buildroot.

# Detailed Explanation

SymbiFlow works with some Linux-enabled SoCs like LiteX/VexRiscv on some platform and work is under way to enable more. Targeting Lattice ECP5, write a tutorial that lets people build their own Buildroot Linux, a bitstream that can run it, and then program an FPGA to see it in practice. As a second track, enable the same software to be run in Renode without modification if someone does not have an FPGA yet wants to work on the software side of things.

# Knowledge Prerequisites

* Sphinx
* bash
* knowledge of Buildroot/Linux kernel will be useful