The controller's STM32 is the hardware behind the buttons, sticks, and display. It is a separate piece of hardware from the IMX linux computers. When the solo-builder runs to compile the full copter and controller images, this Artoo STM32 firmware is included. You can also run the Artoo STM32 builder here separately if you only want to make changes to it. It is fast and simple compared to the entire copter and controller builds.
Some compiled versions of the Artoo STM32 firmware can be found in the /Firmware/Artoo STM32 directory.
Go to the Open Solo github Wiki for Open Solo documentation and instructions