stm32/machine_uart: Implement uart.flush() and uart.txdone().

Since uart.write() of the STM32 port waits until all bytes have
been sent, uart.flush() and uart.txdone() are implemented as empty
functions to provide API consistency.

uart.flush()

flush() will always return immediately.

ret = uart.txdone()

uart.txdone() will always return True.

ports/stm32/machine_uart.c

@@ -523,6 +523,12 @@ STATIC mp_obj_t pyb_uart_irq(size_t n_args, const mp_obj_t *pos_args, mp_map_t *
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_KW(pyb_uart_irq_obj, 1, pyb_uart_irq);
 
+// Since uart.write() waits up to the last byte, uart.txdone() always returns True.
+STATIC mp_obj_t machine_uart_txdone(mp_obj_t self_in) {
+    return mp_const_true;
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_1(machine_uart_txdone_obj, machine_uart_txdone);
+
 STATIC const mp_rom_map_elem_t pyb_uart_locals_dict_table[] = {
     // instance methods
 
@@ -538,11 +544,13 @@ STATIC const mp_rom_map_elem_t pyb_uart_locals_dict_table[] = {
     { MP_ROM_QSTR(MP_QSTR_readinto), MP_ROM_PTR(&mp_stream_readinto_obj) },
     /// \method write(buf)
     { MP_ROM_QSTR(MP_QSTR_write), MP_ROM_PTR(&mp_stream_write_obj) },
+    { MP_ROM_QSTR(MP_QSTR_flush), MP_ROM_PTR(&mp_stream_flush_obj) },
     { MP_ROM_QSTR(MP_QSTR_irq), MP_ROM_PTR(&pyb_uart_irq_obj) },
 
     { MP_ROM_QSTR(MP_QSTR_writechar), MP_ROM_PTR(&pyb_uart_writechar_obj) },
     { MP_ROM_QSTR(MP_QSTR_readchar), MP_ROM_PTR(&pyb_uart_readchar_obj) },
     { MP_ROM_QSTR(MP_QSTR_sendbreak), MP_ROM_PTR(&pyb_uart_sendbreak_obj) },
+    { MP_ROM_QSTR(MP_QSTR_txdone), MP_ROM_PTR(&machine_uart_txdone_obj) },
 
     // class constants
     { MP_ROM_QSTR(MP_QSTR_RTS), MP_ROM_INT(UART_HWCONTROL_RTS) },
@@ -635,6 +643,9 @@ STATIC mp_uint_t pyb_uart_ioctl(mp_obj_t self_in, mp_uint_t request, uintptr_t a
         if ((flags & MP_STREAM_POLL_WR) && uart_tx_avail(self)) {
             ret |= MP_STREAM_POLL_WR;
         }
+    } else if (request == MP_STREAM_FLUSH) {
+        // Since uart.write() waits up to the last byte, uart.flush() always succeds.
+        ret = 0;
     } else {
         *errcode = MP_EINVAL;
         ret = MP_STREAM_ERROR;