Add DFU signature support?

[mithro]

Dfu-util complains with the follow error;

Invalid DFU suffix signature
A valid DFU suffix will be required in a future dfu-util release!!!

Does foboot need to do anything or just yet code which generates the DFU image?

[mithro]

dfu-suffix -v 1209 -p 70b1 -a blink.bin
dfu-util -D blink.dfu

[xobs]

As a side note, here's a pure-Python implementation of a thing that adds the suffix: https://github.com/adafruit/circuitpython/blob/main/tools/dfu.py