DPC compatibility improvements

This build contains several changes to DPC emulation that should improve the compatibility with consoles that have weird bus timings.

Installation:

Download update.bin to your SD card and launch the update from the menu. Don't switch off your 2600 until the screen says "UPDATE COMPLETE".

If you have a ST-LINK/V2 programmer, you can also flash firmware.bin / firmware.hex directly.

This release identifies as R.EDWARDS 13 in the menu.