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.
diff --git a/ports/esp32/mpconfigport.h b/ports/esp32/mpconfigport.h
index 72ecfad..7c09bdb 100644
--- a/ports/esp32/mpconfigport.h
+++ b/ports/esp32/mpconfigport.h
@@ -161,7 +161,7 @@
 #define MICROPY_PY_FRAMEBUF                 (1)
 #define MICROPY_PY_USOCKET_EVENTS           (MICROPY_PY_WEBREPL)
 #define MICROPY_PY_BLUETOOTH_RANDOM_ADDR    (1)
-#define MICROPY_PY_BLUETOOTH_DEFAULT_NAME   ("ESP32")
+#define MICROPY_PY_BLUETOOTH_DEFAULT_GAP_NAME ("ESP32")
 
 // fatfs configuration
 #define MICROPY_FATFS_ENABLE_LFN            (1)