lwip2 updates: no more git sub-sub-module deps, faster checksum, backlog limitation and other fixes (#6887)

* upstream lwIP is now downloaded by a makefile, not subsubmoduled

* lwip2: upstream lwIP not sub-sub-modules anymore
lwip2: Allow IPv4 and IPv6 DNS and SNTP server configured via DHCP to co-exist (patch against upstream)

* lwip2: enable tcp-listen-with-backlog feature

* lwip2 submodule update:
- enable more efficient chksum algorithm thanks to Richard Allen
- enable tcp listener with backlog

* more comments, fix backlog management, fix API
* move default value definition in .cpp
because one must not believe it can be redefined before including WiFiServer.h

* improved backlog handling, it is no more a breaking change

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:1.2-16-ge23a07e"
+#define LWIP_HASH_STR "STABLE-2_1_2_RELEASE/glue:1.2-29-gb543b1f"
 #endif // LWIP_HASH_H

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.
@@ -3573,7 +3579,7 @@ extern "C" {
 #define SNTP_SUPPRESS_DELAY_CHECK 1
 #define SNTP_UPDATE_DELAY_DEFAULT 3600000   // update delay defined by a default weak function
 #define SNTP_UPDATE_DELAY sntp_update_delay_MS_rfc_not_less_than_15000()
-extern uint32_t SNTP_UPDATE_DELAY;
+uint32_t SNTP_UPDATE_DELAY;
 
 #if LWIP_FEATURES
 // esp8266/arduino/lwip-1.4 had 3 possible SNTP servers (constant was harcoded)
@@ -3591,7 +3597,7 @@ extern uint32_t SNTP_UPDATE_DELAY;
 #define SNTP_STARTUP_DELAY 1                // enable startup delay
 #define SNTP_STARTUP_DELAY_FUNC_DEFAULT 0   // to 0 by default via a default weak function
 #define SNTP_STARTUP_DELAY_FUNC sntp_startup_delay_MS_rfc_not_less_than_60000()
-extern uint32_t SNTP_STARTUP_DELAY_FUNC;
+uint32_t SNTP_STARTUP_DELAY_FUNC;
 
 /*
    --------------------------------------------------
@@ -3611,7 +3617,7 @@ struct netif;
 #error LWIP_ERR_T definition should come from lwip1.4 from espressif
 #endif
 //#define LWIP_ERR_T s8
-LWIP_ERR_T lwip_unhandled_packet (struct pbuf* pbuf, struct netif* netif) __attribute__((weak));
+LWIP_ERR_T lwip_unhandled_packet (struct pbuf* pbuf, struct netif* netif);
 
 /*
    --------------------------------------------------