fixed Makefile lesson 14

14-checkpoint/Makefile

@@ -43,4 +43,4 @@ debug: os-image.bin kernel.elf
 
 clean:
 	rm -rf *.bin *.dis *.o os-image.bin *.elf
-	rm -rf kernel/*.o boot/*.bin drivers/*.o
+	rm -rf kernel/*.o boot/*.bin drivers/*.o boot/*.o