ports: fomu: build dfu as part of Makefile
ports: fomu: build dfu as part of Makefile

Now that we have a replacement for `dfu-suffix`, construct a dfu
image rather than a UF2 image.

Signed-off-by: Sean Cross <d7e19930cc1f42c2d0781f4d9e6f1fe5891bf9cf@xobs.io>

adafruit/circuitpython,adafruit/micropython,adafruit/micropython,adafruit/circuitpython,adafruit/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/micropython,adafruit/micropython