ezusb-iidx: Add macro to enable D01 board emulation

Used for dumping firmware, only. C02 and D01 boards only differ
in the FPGA firmware written to the board on boot up. This doesn't
matter for emulation, but when using a real IO board with
iidxio-ezusb.dll, a different FPGA firmware is required to be
flashed in order to get working turntables, top unit lights etc

src/main/ezusb-iidx-emu/msg.c

@@ -160,6 +160,10 @@ static HRESULT ezusb_iidx_emu_msg_interrupt_read(struct iobuf *read)
     otherwise the game's fpga check will fail */
     msg_resp->fpga2_check_flag_unkn = 2;
 
+#ifdef EZUSB_IIDX_EMU_D01_BOARD
+    msg_resp->inverted_pad &= ~(1 << 4);
+#endif
+
     read->pos = sizeof(*msg_resp);
 
     return S_OK;

src/main/ezusb-iidx/msg.h

@@ -59,7 +59,9 @@ struct ezusb_iidx_msg_interrupt_read_packet {
       1: Not used
       2: Not used
       3: Not used
-      4: Not used
+      4: C02/D01 board identifier, 1 = C02, 0 = D01 (on active low). This defines how the game has
+         to flash the FPGA board since D01 needs a different firmware (see result of FPGA CHECK on
+         game bootup)
       5: Not used
       6: usb mute?
       7: Not used