This piece of code is designed to run on an nRF52840-DK (PCA10056). DFU is transfered via UART, you can view the debug output in JLinkRTTViewer.
To use it you need to set the path to your nRF5-SDK-17 in line 5 and the location of your dfu_public_key.c in line 14 of the Makefile. Compile it using make and flash with make flash_mbr flash. Ofcourse you need to flash a bootloader to your DK (if you want to use the BLE Bootloader you need to adapt the FLASH memory location in dfu_in_app_blinky_gcc_nrf52.ld). You can run the firmware update using nrfutil or make serialupdate, that will generate the dfu zip file and send it to /dev/ttyACM0. The serial port can be changed in line 254 of the Makefile