extmod/modbluetooth: Implement config getter for BLE rxbuf size.

Knowing the buffer size can be important, to ensure that valid data will be
received.

docs/library/ubluetooth.rst

@@ -49,7 +49,7 @@ Configuration
       (e.g. PYBD) then it will be returned. Otherwise (e.g. ESP32) a random
       address will be generated when the BLE interface is made active.
 
-    - ``'rxbuf'``: Set the size in bytes of the internal buffer used to store
+    - ``'rxbuf'``: Get/set the size in bytes of the internal buffer used to store
       incoming events.  This buffer is global to the entire BLE driver and so
       handles incoming data for all events, including all characteristics.
       Increasing this allows better handling of bursty incoming data (for

extmod/modbluetooth.c

@@ -293,6 +293,8 @@ STATIC mp_obj_t bluetooth_ble_config(size_t n_args, const mp_obj_t *args, mp_map
                 mp_bluetooth_get_device_addr(addr);
                 return mp_obj_new_bytes(addr, MP_ARRAY_SIZE(addr));
             }
+            case MP_QSTR_rxbuf:
+                return mp_obj_new_int(self->ringbuf.size);
             default:
                 mp_raise_ValueError("unknown config param");
         }