tcpip.h, tcpip.c, api.h, api_lib.c, api_msg.c, sockets.c: replace the field netconn::mbox (sys_mbox_t) per netconn::sem (sys_sem_t) for the task #7490 "Add return value to sys_mbox_post".

src/include/lwip/api.h

@@ -118,9 +118,8 @@ struct netconn {
   } pcb;
   /** the last error this netconn had */
   err_t err;
-  /** mbox that is used mutex-like to synchroneously execute functions
-      in the core context */
-  sys_mbox_t mbox;
+  /** sem that is used to synchroneously execute functions in the core context */
+  sys_sem_t sem;
   /** mbox where received packets are stored until they are fetched
       by the netconn application thread (can grow quite big) */
   sys_mbox_t recvmbox;

src/include/lwip/tcpip.h

@@ -59,7 +59,7 @@ extern sys_sem_t lock_tcpip_core;
 #define LOCK_TCPIP_CORE()
 #define UNLOCK_TCPIP_CORE()
 #define TCPIP_APIMSG(m)       tcpip_apimsg(m)
-#define TCPIP_APIMSG_ACK(m)   sys_mbox_post(m->conn->mbox, NULL)
+#define TCPIP_APIMSG_ACK(m)   sys_sem_signal(m->conn->sem)
 #define TCPIP_NETIFAPI(m)     tcpip_netifapi(m)
 #define TCPIP_NETIFAPI_ACK(m) sys_sem_signal(m->sem)
 #endif /* LWIP_TCPIP_CORE_LOCKING */