esp8266: Make destination for vendor OS debug output soft-configurable.

Use esp.osdebug(None) to disable, or esp.osdebug(uart_id) to send output
to a UART.
diff --git a/esp8266/uart.h b/esp8266/uart.h
index ed0dcfb..2f762db 100644
--- a/esp8266/uart.h
+++ b/esp8266/uart.h
@@ -93,5 +93,6 @@
 int uart0_rx(void);
 void uart_tx_one_char(uint8 uart, uint8 TxChar);
 void uart_flush(uint8 uart);
+void uart_os_config(int uart);
 
 #endif // _INCLUDED_UART_H_