esp8266/machine_uart: Add rxbuf keyword arg to UART constructor/init.

As per the machine.UART documentation, this is used to set the length of
the UART RX buffer.
diff --git a/ports/esp8266/uart.h b/ports/esp8266/uart.h
index 684689a..0e67783 100644
--- a/ports/esp8266/uart.h
+++ b/ports/esp8266/uart.h
@@ -6,6 +6,8 @@
 #define UART0 (0)
 #define UART1 (1)
 
+#define UART0_STATIC_RXBUF_LEN (16)
+
 typedef enum {
     UART_FIVE_BITS = 0x0,
     UART_SIX_BITS = 0x1,
@@ -91,6 +93,8 @@
     int               buff_uart_no;  //indicate which uart use tx/rx buffer
 } UartDevice;
 
+extern uint8 uart_ringbuf_array[UART0_STATIC_RXBUF_LEN];
+
 void uart_init(UartBautRate uart0_br, UartBautRate uart1_br);
 int uart0_rx(void);
 bool uart_rx_wait(uint32_t timeout_us);
@@ -99,6 +103,8 @@
 void uart_flush(uint8 uart);
 void uart_os_config(int uart);
 void uart_setup(uint8 uart);
+int uart0_get_rxbuf_len(void);
+void uart0_set_rxbuf(uint8 *buf, int len);
 // check status of rx/tx
 int uart_rx_any(uint8 uart);
 int uart_tx_any_room(uint8 uart);