stm32/Makefile: Make the generation of `firmware.bin` explicit.
stm32/Makefile: Make the generation of `firmware.bin` explicit.

The file `$(BUILD)/firmware.bin` was used by the target `deploy-stlink` and
`deploy-openocd` but it was generated indirectly by the target
`firmware.dfu`.

As this file could be used to program boards directly by a Mass Storage
copy, it's better to make it explicitly generated.

Additionally, some target are refactored to remove redundancy and be more
explicit on dependencies.

henriknelson/micropython,adafruit/circuitpython,henriknelson/micropython,bvernoux/micropython,bvernoux/micropython,bvernoux/micropython,MrSurly/micropython,MrSurly/micropython,MrSurly/micropython,henriknelson/micropython,bvernoux/micropython,adafruit/circuitpython,MrSurly/micropython,adafruit/circuitpython,MrSurly/micropython,bvernoux/micropython,henriknelson/micropython,adafruit/circuitpython,adafruit/circuitpython,henriknelson/micropython,adafruit/circuitpython