add a cleanup task.

os/Makefile

	@echo "make help  - display this message"
	@echo "make build - build the os"
	@echo "make run   - run the os in qemu"
	@echo "make clean - clean up"
build: first.flp
run: first.flp
	qemu-system-i386 -fda first.flp
clean:
	rm -f *.flp *.bin mikeos*.zip