DIY Game Boy Cartridge

Purpose: build a GB cart that can be flashed and has enough SRAM to be useful with LSDJ.

Preliminary research:
 * http://www.reinerziegler.de/readplus.htm
 * https://chipmusic.org/forums/topic/14683/question-about-diy-gb-flashcarts-lsdj-and-sram/ - original SRAM struggle