Awesome
ortho60-keyberon 
Keyberon port to the Cannon Keys Ortho60
Installing the precompiled firmware
Download the precompiled firmware and install it as described in the official documentation.
Sort version: while the keyboard is connected to USB, press the reset button on the blue pill. The green LED should blink. In a terminal, type
sudo dfu-util -d 1eaf:0003 -a 2 -D ortho60-keyberon.bin
Push reset and you're ready to go.
Installing a custom firmware
First install the rust toolchain by typing, as user, these 4 commands:
curl https://sh.rustup.rs -sSf | sh
rustup target add thumbv7m-none-eabi
rustup component add llvm-tools-preview
cargo install cargo-binutils
Then build:
git clone https://github.com/TeXitoi/ortho60-keyberon
cd ortho60-keyberon
# edit src/layout.rs to customize your layout
cargo objcopy --bin ortho60-keyberon --release -- -O binary ortho60-keyberon.bin
Now flash the generated ortho60-keyberon.bin file by following the instructions in the previous section.
Notes
reflash bootloader:
openocd -f interface/stlink-v2.cfg -f target/stm32f1x.cfg -c "init; reset halt; stm32f1x mass_erase 0; program generic_boot20_pc13.bin exit 0x08000000"
Cannonkeys flash procedure: https://docs.cannonkeys.com/flashing/