lwip2 submodule update:

- enable more efficient chksum algorithm thanks to Richard Allen - enable tcp listener with backlog
esp8266 · d-a-v · Feb 18, 2020 · Dec 7, 2019 · Dec 7, 2019 · Jan 23, 2020
commit e2a72b49bfea72c19d1b8f0982b69265da93460f
diff --git a/tools/sdk/lib/liblwip2-1460-feat.a b/tools/sdk/lib/liblwip2-1460-feat.a
diff --git a/tools/sdk/lib/liblwip2-1460.a b/tools/sdk/lib/liblwip2-1460.a
diff --git a/tools/sdk/lib/liblwip2-536-feat.a b/tools/sdk/lib/liblwip2-536-feat.a
diff --git a/tools/sdk/lib/liblwip2-536.a b/tools/sdk/lib/liblwip2-536.a
diff --git a/tools/sdk/lib/liblwip6-1460-feat.a b/tools/sdk/lib/liblwip6-1460-feat.a
diff --git a/tools/sdk/lib/liblwip6-536-feat.a b/tools/sdk/lib/liblwip6-536-feat.a
diff --git a/tools/sdk/lwip2/builder b/tools/sdk/lwip2/builder
diff --git a/tools/sdk/lwip2/include/lwip-git-hash.h b/tools/sdk/lwip2/include/lwip-git-hash.h
@@ -1,5 +1,5 @@
 // generated by makefiles/make-lwip2-hash
 #ifndef LWIP_HASH_H
 #define LWIP_HASH_H
-#define LWIP_HASH_STR "STABLE-2_1_2_RELEASE/glue:1.2-24-g25fb4cb"
+#define LWIP_HASH_STR "STABLE-2_1_2_RELEASE/glue:1.2-27-g7aade53"
 #endif // LWIP_HASH_H
diff --git a/tools/sdk/lwip2/include/lwipopts.h b/tools/sdk/lwip2/include/lwipopts.h
@@ -1402,7 +1402,7 @@
  * TCP_LISTEN_BACKLOG: Enable the backlog option for tcp listen pcb.
  */
 #if !defined TCP_LISTEN_BACKLOG || defined __DOXYGEN__
-#define TCP_LISTEN_BACKLOG              0
+#define TCP_LISTEN_BACKLOG              LWIP_FEATURES // 0
 #endif
 
 /**
@@ -2278,6 +2278,12 @@
  * @ingroup lwip_opts_infrastructure
  * @{
  */
+/**
+ * LWIP_CHKSUM_ALGORITHM==3: Checksum algorithm fastest for ESP8266
+ */
+#if !defined LWIP_CHKSUM_ALGORITHM || defined __DOXYGEN__
+#define LWIP_CHKSUM_ALGORITHM           3 // 2
+#endif
 /**
  * LWIP_CHECKSUM_CTRL_PER_NETIF==1: Checksum generation/check can be enabled/disabled
  * per netif.