Allow png->lz compression in the build process

To enable, append '${IMGFILES}' to line 19:
pokecrystal.o: pokecrystal.asm constants.asm wram.asm ${TEXTFILES} ${IMGFILES}

Also append '${LZFILES}' to line 34:
	rm -f main.tx pokecrystal.o pokecrystal.gbc ${TEXTFILES} ${LZFILES}