core: 'make dfu' waits for bootloader to start

qmk/qmk_firmware@0b8c448

Author: tmk

rules.mk

@@ -420,6 +420,10 @@ flip: $(TARGET).hex
 	batchisp -hardware usb -device $(MCU) -operation start reset 0
 
 dfu: $(TARGET).hex
+	@until dfu-programmer atmega32u4 get bootloader-version > /dev/null 2>&1; do \
+		echo  -n "."; \
+		sleep 1; \
+	done
 ifneq (, $(findstring 0.7, $(shell dfu-programmer --version 2>&1)))
 	dfu-programmer $(MCU) erase --force
 else