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[USB] Support DFU runtime protocol along CDC
This adds support for the DFU runtime protocol, which allows resetting into the bootloader using a DFU command. This allows e.g. dfu-util to handle the complete firmware upload, including the needed reset. This consists of a number of changes: - An extra interface is added to the USB configuration descriptor. This descriptor has two parts (interface descriptor and functional descriptor) which together indicate to a host that this device supports DFU. - Control packets to this new interface are detected by the CDC code an forwarded to a new USBD_DFU_Runtime_Control() function. - This new function handles the DFU GET_STATE, GET_STATUS and DFU_DETACH commands. The former are optional, but simple enough, the latter is mandatory and handles resetting into the bootloader. - The CDC device descriptor is changed to become a composite device (CDC and DFU). This allows operating systems (in particular Windows, Linux did not really need this) to identify two different subdevices, and install different drivers for each (on Windows, this is serusb for the CDC part and WinUSB/libusb for the DFU part). Without this, dfu-util on Windows could not access the DFU commands when the serial driver was loaded. Because the CDC functionality already exposes two interfaces (which together form a single serial port), an IAD (Interface Association Descriptor) is inserted before these interfaces to group them together in a single subdevice. No IAD is needed for the DFU interface, since it is just a single interface. To become a composite device, the device class must be changed from CDC to a composite device class. This was originally class 0/0/0, but together with the IAD, a new EF/2/1 deviceclass was also introduced, which is used now. Note that this only adds descriptors and a command handler on the default control endpoint, so no extra (scarce) endpoints are used by this, just a bit of memory. This commit is still a bit rough, because: - The DFU descriptors and code are now pulled in directly by the CDC code (and HID is not supported yet). Ideally, there should be some kind of pluggable USB library where different interfaces can be registered independent of each other (see also stm32duino#687). - The interface number is hardcoded in the DFU descriptor. - The reset to bootloader happens immediately, while it might be better to wait a short while to allow the current USB transaction to complete. - DFU support is unconditionally advertised, while not all boards might support DFU.
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