Custom path support (#1333)

also including:
* getting rid of shitty strings
* all new, cleaner ROM handling code
* base for DSi savestates
* GBA slot addons (for now, memory cart)

src/SPI.h

@@ -28,6 +28,7 @@ void SetupDirectBoot(bool dsi);
 
 u32 FixFirmwareLength(u32 originalLength);
 
+u32 GetFirmwareLength();
 u8 GetConsoleType();
 u8 GetWifiVersion();
 u8 GetNWifiVersion();