Allow uploading using DFU to ChibiOS-only boards

ArduPilot is being ported to a growing number of smaller flight-controllers thanks to ChibiOS.

I've created a [wiki page that explains the easiest method to get ArduPilot onto the board](http://ardupilot.org/copter/docs/common-loading-firmware-onto-chibios-only-boards.html) (the first time) but it involves using the Betaflight Configurator. It's a fine tool but it would be nice if the Install Firmware screen would allow uploading ArduPilot using dfu.

@tridge probably knows better but I think there are at least two possible ways it could be fit into the Install Firmware screen:

- extend the "Force PX4 Bootloader" so that it allows writing the appropriate ardupilot compatible bootloader to the board. After this is done then the existing upload procedure should work.
- extend the "Load custom firmware" to feature to allow loading the "arduXXX_with_bl.hex" file

Start in the Mission Planner Install Firmware screen and inspect the existing “Force PX4 Bootloader” and “Load custom firmware” flows. Determine how DFU devices and ardupilot-compatible bootloader or *_with_bl.hex files are currently handled. Done means a supported ChibiOS-only board can be uploaded through the Install Firmware screen without Betaflight Configurator, with coverage for the selected flow.

Written by the indexing model from the issue text.