Add 128KB bankswitching (V-INT pin) support

[technobly]

Feature

Add 128KB bankswitching (V-INT pin) support. We have the pin hooked up already, but need to implement the bankswitching firmware. Might require the STM32F412 or STM32F405 though. This is primarily for Malban's Vectorblade game.

Add design notes below please!