LWIP_TCPIP_CORE_LOCKING macro

LWIP_TCPIP_CORE_LOCKING Creates a global mutex that is held during TCPIP thread operations. Can be locked by client code to perform lwIP operations without changing into TCPIP thread using callbacks. See LOCK_TCPIP_CORE() and UNLOCK_TCPIP_CORE(). Your system should provide mutexes supporting priority inversion to use this.