Clarify python2 call

Nixie_Firmware_Mbed/Makefile

@@ -515,7 +515,7 @@ $(PROJECT).hex: $(PROJECT).elf
 	$(ELF2BIN) -O ihex $< $@
 
 $(PROJECT).dfu: $(PROJECT).bin
-	python $(VPATH)/dfu.py -b 0x08000000:$< -D 0x0483:0xdf11 $@
+	python2 $(VPATH)/dfu.py -b 0x08000000:$< -D 0x0483:0xdf11 $@
 	+@echo "===== dfu file ready to flash: $(OBJDIR)/$@ ====="