Require `BOOTLOADER = qmk-dfu` for `:bootloader` target (#12136)

tmk_core/avr.mk

@@ -283,6 +283,9 @@ extcoff: $(BUILD_DIR)/$(TARGET).elf
 	$(COFFCONVERT) -O coff-ext-avr $< $(BUILD_DIR)/$(TARGET).cof
 
 bootloader:
+ifneq ($(strip $(BOOTLOADER)), qmk-dfu)
+	$(error Please set BOOTLOADER = qmk-dfu first!)
+endif
 	make -C lib/lufa/Bootloaders/DFU/ clean
 	bin/qmk generate-dfu-header --quiet --keyboard $(KEYBOARD) --output lib/lufa/Bootloaders/DFU/Keyboard.h
 	$(eval MAX_SIZE=$(shell n=`$(CC) -E -mmcu=$(MCU) $(CFLAGS) $(OPT_DEFS) tmk_core/common/avr/bootloader_size.c 2> /dev/null | sed -ne 's/\r//;/^#/n;/^AVR_SIZE:/,$${s/^AVR_SIZE: //;p;}'` && echo $$(($$n)) || echo 0))