usbd: Implement usb backend system (#3737)

* initial impl

* reviews

* upstreamed deReaperJosh changes

* fixed config.cpp

---------

Co-authored-by: georgemoralis <giorgosmrls@gmail.com>

src/core/libraries/usbd/usbd.h

@@ -4,6 +4,7 @@
 #pragma once
 
 #include "common/types.h"
+#include "usb_backend.h"
 
 extern "C" {
 struct libusb_device;
@@ -21,6 +22,8 @@ class SymbolsResolver;
 
 namespace Libraries::Usbd {
 
+extern std::shared_ptr<UsbBackend> usb_backend;
+
 using SceUsbdDevice = libusb_device;
 using SceUsbdDeviceHandle = libusb_device_handle;
 using SceUsbdDeviceDescriptor = libusb_device_descriptor;
@@ -29,6 +32,11 @@ using SceUsbdTransfer = libusb_transfer;
 using SceUsbdControlSetup = libusb_control_setup;
 using SceUsbdTransferCallback = void PS4_SYSV_ABI (*)(SceUsbdTransfer* transfer);
 
+// TODO: implement emulated devices
+using SkylandersPortalBackend = UsbRealBackend;
+using InfinityBaseBackend = UsbRealBackend;
+using DimensionsToypadBackend = UsbRealBackend;
+
 enum class SceUsbdSpeed : u32 {
     UNKNOWN = 0,
     LOW = 1,