The RAK811 Trackerboard thread i getting quite big.
If you want to add hardware you might want to consider using the Arduino or mbed frameworks.
This guy is working on an Arduino port, https://github.com/stm32duino/Arduino_Core_STM32
Also some people are looking at the mbed framework that recently has added support for the RAK811 board.
However using the mbed framework with OS fills the entire flash and not the basic support for the gps and sensor. I would like to get the people interested in this to write in this thread.
I will do my best to get the GPS and mems sensor working here,