esp8266: Add module weak link from json to ujson.

esp8266/mpconfigport.h

@@ -102,6 +102,7 @@ extern const struct _mp_obj_module_t mp_module_machine;
 #define MICROPY_PORT_BUILTIN_MODULE_WEAK_LINKS \
     { MP_OBJ_NEW_QSTR(MP_QSTR_time), (mp_obj_t)&utime_module }, \
     { MP_OBJ_NEW_QSTR(MP_QSTR_os), (mp_obj_t)&uos_module }, \
+    { MP_OBJ_NEW_QSTR(MP_QSTR_json), (mp_obj_t)&mp_module_ujson }, \
 
 #define MP_STATE_PORT MP_STATE_VM
 

esp8266/qstrdefsport.h

@@ -163,3 +163,5 @@ Q(mode)
 Q(period)
 Q(ONE_SHOT)
 Q(PERIODIC)
+
+Q(json)