lk-overlay contains many different projects

using the vc4-stage1, vc4-stage2, and rpi2-test projects together, you can do the entire boot chain (dram init and loading linux) using open source code

other projects act as demo's or tests on how to run custom code at various stages, but not actually boot linux on the arm core many of those demos work on the the entire pi model range

pi3 support is only broken due to arm side problems, which could be fixed by just using a different bootloader

and the https://github.com/librerpi/rpi-open-firmware codebase can already boot linux headlessly on both pi2 and pi3, it uses a different arm bootloader