The variable `DFU_USB_ID` is used by `dfu-util` to specify the device to be flashed. The STM32 make system prevents `dfu-util` from being used as programmer if this variable is not set, although `dfu-util.mk` generates a default value for this variable from the `USB_VID` and `USB_PID` variables if necessary. Therefore, checking the `DFU_USB_ID` variable is removed here. If a board requires a specific combination of VID/PID for `dfu_util`, it is responsible for setting the `DFU_USB_ID` variable.
Allow issuing a reset to bootloader sequence by abusing the RTS and
the DTR pins of a TTL adapter. This makes flashing via UART much
more convenient, as no jumpers need to be placed to select booting to
the bootloader / flash and no reset buttons need to be pressed.
