LWIP_ASSERT_CORE_LOCKED macro

Macro/function to check whether lwIP's threading/locking requirements are satisfied during current function call. This macro usually calls a function that is implemented in the OS-dependent sys layer and performs the following checks: - Not in ISR (this should be checked for NO_SYS==1, too!) - If LWIP_TCPIP_CORE_LOCKING = 1: TCPIP core lock is held - If LWIP_TCPIP_CORE_LOCKING = 0: function is called from TCPIP thread @see multithreading