esp32/machine_uart: Release GIL for blocking reads.

If we're reading from an UART with a non-zero timeout, we can release the
GIL so that other threads/tasks may run while we are sleeping waiting for
data to arrive.

Signed-off-by: Daniël van de Giessen <daniel@dvdgiessen.nl>

ports/esp32/machine_uart.c

@@ -472,8 +472,17 @@ STATIC mp_uint_t machine_uart_read(mp_obj_t self_in, void *buf_in, mp_uint_t siz
         time_to_wait = pdMS_TO_TICKS(self->timeout);
     }
 
+    bool release_gil = time_to_wait > 0;
+    if (release_gil) {
+        MP_THREAD_GIL_EXIT();
+    }
+
     int bytes_read = uart_read_bytes(self->uart_num, buf_in, size, time_to_wait);
 
+    if (release_gil) {
+        MP_THREAD_GIL_ENTER();
+    }
+
     if (bytes_read <= 0) {
         *errcode = MP_EAGAIN;
         return MP_STREAM_ERROR;