Document how to load and run zephyr RTOS

[WorldofJARcraft]

Starting with the upcoming release 4.2.0 (and the current main branch), the zephyr RTOS has support for running on the 32/64 bit configurations of cva6 on the Genesys2 board. This PR adds instructions for building and loading zephyr for these configurations to the README. It also adds a script that can be used to load zephyr from an SD card. I also have automated scripts that setup the build dependencies for zephyr and build a sample application - I can add them to the PR and add a corresponding GitHub action if you are interested. This closes #2480.