extmod/machine_uart: Factor ports' UART Python bindings to common code.

This is a code factoring to have the Python bindings in one location, and
all the ports use those same bindings.  For all ports except the two listed
below there is no functional change.

The nrf port has UART.sendbreak() removed, but this method previously did
nothing.

The zephyr port has the following methods added:
- UART.init(): supports setting timeout and timeout_char.
- UART.deinit(): does nothing, just returns None.
- UART.flush(): raises OSError(EINVAL) because it's not implemented.
- UART.any() and UART.txdone(): raise NotImplementedError.

Signed-off-by: Damien George <damien@micropython.org>

drivers/cyw43/cywbt.c

@@ -49,7 +49,7 @@ extern uint8_t mp_bluetooth_hci_cmd_buf[4 + 256];
 #include "uart.h"
 
 // Provided by the port.
-extern pyb_uart_obj_t mp_bluetooth_hci_uart_obj;
+extern machine_uart_obj_t mp_bluetooth_hci_uart_obj;
 
 STATIC void cywbt_wait_cts_low(void) {
     mp_hal_pin_config(CYW43_PIN_BT_CTS, MP_HAL_PIN_MODE_INPUT, MP_HAL_PIN_PULL_UP, 0);