extmod/modbluetooth: Rename module to "ubluetooth".

For consistency with "umachine". Now that weak links are enabled
by default for built-in modules, this should be a no-op, but allows
extension of the bluetooth module by user code.

Also move registration of ubluetooth to objmodule rather than
port-specific.

py/objmodule.c

@@ -224,6 +224,9 @@ STATIC const mp_rom_map_elem_t mp_builtin_module_table[] = {
 #if MICROPY_PY_BTREE
     { MP_ROM_QSTR(MP_QSTR_btree), MP_ROM_PTR(&mp_module_btree) },
 #endif
+#if MICROPY_PY_BLUETOOTH
+    { MP_ROM_QSTR(MP_QSTR_ubluetooth), MP_ROM_PTR(&mp_module_ubluetooth) },
+#endif
 
     // extra builtin modules as defined by a port
     MICROPY_PORT_BUILTIN_MODULES