makefiles/boards/stm32.ink.mk: use USB_VID/USB_PID for DFU_USB_ID

The variable `DFU_USB_ID` is used to specify the device to be used by `dfu-util`. The STM32 make system rejects the use of `dfu-util` as programmer if this variable is not set. This in turn prevents the use of USBUS DFU without setting the variable, although `dfu-util.mk` can generate the required variable `DFU_USB_ID` from `USB_VID` and `USB_PID` if needed. Therefore the STM32 make system is modified to accept an empty `DFU_USB_ID` variable when using `dfu-util` for USBUS DFU (module `usbus_dfu` is used).

makefiles/boards/stm32.inc.mk

@@ -24,7 +24,7 @@ ifeq (dfu-util,$(PROGRAMMER))
   # with blackmagic_dfu, see https://github.com/blacksphere/blackmagic/
   # To stop bootloader from loading an existing firmware, pull down
   # (ground) GPIO B1.
-  ifeq (,$(DFU_USB_ID))
+  ifeq (,$(DFU_USB_ID) $(filter usbus_dfu,$(USEMODULE)))
     $(error DFU_USB_ID is not set)
   endif
 endif