extmod/machine_usb_device: Add support for Python USB devices.

This new machine-module driver provides a "USBDevice" singleton object and
a shim TinyUSB "runtime" driver that delegates the descriptors and all of
the TinyUSB callbacks to Python functions.  This allows writing arbitrary
USB devices in pure Python.  It's also possible to have a base built-in
USB device implemented in C (eg CDC, or CDC+MSC) and a Python USB device
added on top of that.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

py/runtime.c

@@ -171,6 +171,10 @@ void mp_init(void) {
     MP_STATE_VM(bluetooth) = MP_OBJ_NULL;
     #endif
 
+    #if MICROPY_HW_ENABLE_USB_RUNTIME_DEVICE
+    MP_STATE_VM(usbd) = MP_OBJ_NULL;
+    #endif
+
     #if MICROPY_PY_THREAD_GIL
     mp_thread_mutex_init(&MP_STATE_VM(gil_mutex));
     #endif