To run ./build.sh, you need to install xargo and be using rustup for managing rust installations.
./build.sh [test name] [debug|release]
Copy the generated file ./target/thumbv5te-none-eabi/<relmode>/crossbar9.firm to your 3DS's SD card, and run using boot9strap.
Temporarily borrowing Decrypt9WIP as a submodule (for the functioning filesystem code). Thanks, d0k3!