nrf/modules/machine/uart: Implement uart.flush() and uart.txdone().

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

uart.flush()

flush() will always return immediately, even if the last byte
may still be sent.

ret = uart.txdone()

uart.txdone() will always return True, even if the last byte
may still be sent.

ports/nrf/modules/machine/uart.c

@@ -292,6 +292,12 @@ STATIC mp_obj_t machine_hard_uart_sendbreak(mp_obj_t self_in) {
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(machine_hard_uart_sendbreak_obj, machine_hard_uart_sendbreak);
 
+// 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 machine_hard_uart_locals_dict_table[] = {
     // instance methods
     { MP_ROM_QSTR(MP_QSTR_read), MP_ROM_PTR(&mp_stream_read_obj) },
@@ -302,6 +308,8 @@ STATIC const mp_rom_map_elem_t machine_hard_uart_locals_dict_table[] = {
     { MP_ROM_QSTR(MP_QSTR_writechar), MP_ROM_PTR(&machine_hard_uart_writechar_obj) },
     { MP_ROM_QSTR(MP_QSTR_readchar), MP_ROM_PTR(&machine_hard_uart_readchar_obj) },
     { MP_ROM_QSTR(MP_QSTR_sendbreak), MP_ROM_PTR(&machine_hard_uart_sendbreak_obj) },
+    { MP_ROM_QSTR(MP_QSTR_flush), MP_ROM_PTR(&mp_stream_flush_obj) },
+    { MP_ROM_QSTR(MP_QSTR_txdone), MP_ROM_PTR(&machine_uart_txdone_obj) },
 
     // class constants
 /*
@@ -347,6 +355,11 @@ STATIC mp_uint_t machine_hard_uart_write(mp_obj_t self_in, const void *buf_in, m
 STATIC mp_uint_t machine_hard_uart_ioctl(mp_obj_t self_in, mp_uint_t request, uintptr_t arg, int *errcode) {
     machine_hard_uart_obj_t *self = self_in;
     (void)self;
+
+    if (request == MP_STREAM_FLUSH) {
+        // Since uart.write() waits up to the last byte, uart.flush() always succeds.
+        return 0;
+    }
     return MP_STREAM_ERROR;
 }