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

tests/host/common/MockWiFiServerSocket.cpp

@@ -60,6 +60,13 @@ int serverAccept (int srvsock)
 
 void WiFiServer::begin (uint16_t port)
 {
+    return begin(port, !0);
+}
+
+void WiFiServer::begin (uint16_t port, uint8_t backlog)
+{
+    if (!backlog)
+        return;
 	_port = port;
 	return begin();
 }
@@ -109,13 +116,13 @@ void WiFiServer::begin ()
 
 
 	// store int into pointer
-	_pcb = int2pcb(sock);
+	_listen_pcb = int2pcb(sock);
 }
 
 bool WiFiServer::hasClient ()
 {
 	struct pollfd p;
-	p.fd = pcb2int(_pcb);
+	p.fd = pcb2int(_listen_pcb);
 	p.events = POLLIN;
 	return poll(&p, 1, 0) && p.revents == POLLIN;
 }
@@ -134,7 +141,7 @@ size_t WiFiServer::write (const uint8_t *buf, size_t size)
 
 void WiFiServer::close ()
 {
-	if (pcb2int(_pcb) >= 0)
-		::close(pcb2int(_pcb));
-	_pcb = int2pcb(-1);
+	if (pcb2int(_listen_pcb) >= 0)
+		::close(pcb2int(_listen_pcb));
+	_listen_pcb = int2pcb(-1);
 }