This avoids a dependency on nrfutil. I have verified that it creates
equivalent zip files to a wasp-os DFU zip file I downloaded here:
https://github.com/wasp-os/wasp-os/releases/
I have also tested that it produces valid DFU files that can be uploaded
using the dfu.py program here to my PineTime:
3d6fd30d33
There are some minor differences in the generated file that should not
matter in practice (JSON whitespace, firmware file name, zip
compression).