esp32/usb: Wake main thread when USB receives data.

This improves (decreases) the latency on stdin, on SoCs with built-in USB
and using TinyUSB, like S2 and S3.

Signed-off-by: Andrew Leech <andrew.leech@planetinnovation.com.au>

ports/esp32/usb.c

@@ -40,8 +40,10 @@
 
 static uint8_t usb_rx_buf[CONFIG_TINYUSB_CDC_RX_BUFSIZE];
 
+// This is called from FreeRTOS task "tusb_tsk" in espressif__esp_tinyusb (not an ISR).
 static void usb_callback_rx(int itf, cdcacm_event_t *event) {
-    // TODO: what happens if more chars come in during this function, are they lost?
+    // espressif__esp_tinyusb places tinyusb rx data onto freertos ringbuffer which
+    // this function forwards onto our stdin_ringbuf.
     for (;;) {
         size_t len = 0;
         esp_err_t ret = tinyusb_cdcacm_read(itf, usb_rx_buf, sizeof(usb_rx_buf), &len);
@@ -58,6 +60,7 @@ static void usb_callback_rx(int itf, cdcacm_event_t *event) {
                 ringbuf_put(&stdin_ringbuf, usb_rx_buf[i]);
             }
         }
+        mp_hal_wake_main_task();
     }
 }