add basic usb install support (see commit message).

transfers seems to work, although i have done very little testing.
i plan to extend the usb script so that it supports normal usb transfers, such as uploading and
downloading files to and from the switch.

however, most users are likely better off using mtp for said transfers.

the usb transfer code was taken from Haze, which is part of Atmosphere.

sphaira/include/yati/source/usb.hpp

@@ -0,0 +1,51 @@
+#pragma once
+
+#include "base.hpp"
+#include "fs.hpp"
+#include <switch.h>
+
+namespace sphaira::yati::source {
+
+struct Usb final : Base {
+    enum { USBModule = 523 };
+
+    enum : Result {
+        Result_BadMagic = MAKERESULT(USBModule, 0),
+        Result_BadVersion = MAKERESULT(USBModule, 1),
+        Result_BadCount = MAKERESULT(USBModule, 2),
+        Result_BadTransferSize = MAKERESULT(USBModule, 3),
+        Result_BadTotalSize = MAKERESULT(USBModule, 4),
+    };
+
+    Usb(u64 transfer_timeout);
+    ~Usb();
+
+    Result Read(void* buf, s64 off, s64 size, u64* bytes_read) override;
+    Result Finished() const;
+
+    Result Init();
+    Result WaitForConnection(u64 timeout, u32& speed, u32& count);
+
+private:
+    enum UsbSessionEndpoint {
+        UsbSessionEndpoint_In = 0,
+        UsbSessionEndpoint_Out = 1,
+    };
+
+    Result SendCommand(s64 off, s64 size) const;
+    Result InternalRead(void* buf, s64 off, s64 size) const;
+
+    bool GetConfigured() const;
+    Event *GetCompletionEvent(UsbSessionEndpoint ep) const;
+    Result WaitTransferCompletion(UsbSessionEndpoint ep, u64 timeout) const;
+    Result TransferAsync(UsbSessionEndpoint ep, void *buffer, u32 size, u32 *out_urb_id) const;
+    Result GetTransferResult(UsbSessionEndpoint ep, u32 urb_id, u32 *out_requested_size, u32 *out_transferred_size) const;
+    Result TransferPacketImpl(bool read, void *page, u32 size, u32 *out_size_transferred, u64 timeout) const;
+
+private:
+    UsbDsInterface* m_interface{};
+    UsbDsEndpoint* m_endpoints[2]{};
+    u64 m_transfer_timeout{};
+};
+
+} // namespace sphaira::yati::source