extmod/modbluetooth: Add support for changing the GAP device name.

This commit allows the user to set/get the GAP device name used by service
0x1800, characteristic 0x2a00.  The usage is:

    BLE.config(gap_name="myname")
    print(BLE.config("gap_name"))

As part of this change the compile-time setting
MICROPY_PY_BLUETOOTH_DEFAULT_NAME is renamed to
MICROPY_PY_BLUETOOTH_DEFAULT_GAP_NAME to emphasise its link to GAP and this
new "gap_name" config value.  And the default value of this for the NimBLE
bindings is changed from "PYBD" to "MPY NIMBLE" to be more generic.

docs/library/ubluetooth.rst

@@ -51,6 +51,10 @@ Configuration
       Note: on some ports, accessing this value requires that the interface is
       active (so that the MAC address can be queried from the controller).
 
+    - ``'gap_name'``: Get/set the GAP device name used by service 0x1800,
+      characteristic 0x2a00.  This can be set at any time and changed multiple
+      times.
+
     - ``'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.