add MEMP_NUM_NETCONN in lwipopts

modules/Ethernet/lwipopts.h

@@ -80,6 +80,12 @@ a lot of data that needs to be copied, this should be set high. */
 #define MEMP_NUM_SYS_TIMEOUT    10
 //#define MEMP_NUM_SYS_TIMEOUT    100
 
+/**
+ * MEMP_NUM_NETCONN: the number of struct netconns.
+ * (only needed if you use the sequential API, like api_lib.c)
+ */
+#define MEMP_NUM_NETCONN                8
+
 /* ---------- Pbuf options ---------- */
 /* PBUF_POOL_SIZE: the number of buffers in the pbuf pool. */
 //#define PBUF_POOL_SIZE          15