Makefile target 'cart' is unimplemented

[gregkrsak]

The default target of the Makefile builds an iNES ROM file. I'd like to also add a physical cart target that prepares the appropriate .bin files for flashing onto a physical cartridge.

I'm not yet familiar with how to do this. I suppose I'll eventually get to it, but if someone wants to jump in and do this / verify it works, feel free.