makefile: create a libarduino.a archive file with when calling make.
This libarduino.a archive shall be linked to your project by adding the
following contents to your project's makefile:

LIBPATH = -L <your path>/lib/libarduino
LIB = -larduino

$(PROG).elf: $(OBJ)
	avr-gcc $(OBJ) $(LIBPATH) $(LIB) $(LFLAGS) -o $(PROG).elf
	               ^^^^^^^^^^^^^^^^^