Compiling

Place NRF5 SDK 12.1 next to this directory (../nRF5_SDK_12.1.0_0d23e2a/)
Set compiler path in ../nRF5_SDK_12.1.0_0d23e2a/components/toolchain/gcc/Makefile.posix
Install python 2.7 and pip install nrfutil (and maybe add ~/.local/bin to $PATH)
Install make
Install gcc-arm-none-eabi
Execute make

BLE firmware update

Install NRF Connect
Connect DFU boot pins
Connect to DFUTARG
Select DFU

Boot pin configuration

BLE DFU mode
RC mode

Debug pins (SWD)

Pictures