multi-mss menus for lwip2, remove lwip xcc variant, lwip2 readme, updates and fixes (#4039)

lwip2 updates:
  > multi-mss makefile
  > forwardported espconn (no multicast yet)
  > restore max 3 ntp servers for configTime() coherency
  > unchain seldom chained pbufs
  > dns cache name length back to (256->48->) 128
  > use sntp_stop/start() when dhcp address got
  > fix netif's hostname glue-handling
  > forwardported ping from lwip1.4
fix #3970
fix maybe #3963

Author: d-a-v

boards.txt

@@ -178,6 +178,7 @@ SECTIONS
     *liblwip_gcc.a:(.literal .text .literal.* .text.*)
     *liblwip_src.a:(.literal .text .literal.* .text.*)
     *liblwip2.a:(.literal .text .literal.* .text.*)
+    *liblwip2_1460.a:(.literal .text .literal.* .text.*)
     *libaxtls.a:(.literal .text .literal.* .text.*)
     *libat.a:(.literal.* .text.*)
     *libcrypto.a:(.literal.* .text.*)

tools/sdk/ld/eagle.app.v6.common.ld

@@ -2,5 +2,7 @@
 all install clean: builder/lwip2-src/README
 	make -C builder -f Makefile.arduino $@
 
+download: builder/lwip2-src/README
+
 builder/lwip2-src/README:
 	git submodule update --init --recursive builder

tools/sdk/lib/liblwip2.a

@@ -0,0 +1,10 @@
+
+make install	download, compile, install lwip2
+make download	download lwIP-2 builder
+make clean	clean builder only
+
+glue and lwIP debug options are in builder/glue/gluedebug.h
+MSS values are in builder/Makefile.arduino
+MSS values in boards.txt are only informative
+
+current lwip2 submodule repository: https://github.com/d-a-v/esp82xx-nonos-linklayer/tree/arduino-2.4.0

tools/sdk/lib/liblwip2_1460.a

@@ -1050,7 +1050,7 @@
 
 /** DNS maximum host name length supported in the name table. */
 #if !defined DNS_MAX_NAME_LENGTH || defined __DOXYGEN__
-#define DNS_MAX_NAME_LENGTH             48 // 256
+#define DNS_MAX_NAME_LENGTH             128 // 256
 #endif
 
 /** The maximum of DNS servers
@@ -1214,9 +1214,9 @@
  * when opening a connection. For the transmit size, this MSS sets
  * an upper limit on the MSS advertised by the remote host.
  */
-#if !defined TCP_MSS || defined __DOXYGEN__
-#define TCP_MSS                         536
-#endif
+//#if !defined TCP_MSS || defined __DOXYGEN__
+//#define TCP_MSS                         536
+//#endif
 
 /**
  * TCP_CALCULATE_EFF_SEND_MSS: "The maximum size of a segment that TCP really
@@ -1312,7 +1312,7 @@
  * TCP_MSS/4: Try to create 4 fragments or less per TCP packet.
  */
 #if !defined TCP_OVERSIZE || defined __DOXYGEN__
-#define TCP_OVERSIZE                    TCP_MSS
+#define TCP_OVERSIZE                    TCP_MSS	// TCP_MSS is STRONGLY SUGGESTED FOR ESP8266 BLOBS !!
 #endif
 
 /**
@@ -1506,7 +1506,7 @@
  * @todo: TCP and IP-frag do not work with this, yet:
  */
 #if !defined LWIP_NETIF_TX_SINGLE_PBUF || defined __DOXYGEN__
-#define LWIP_NETIF_TX_SINGLE_PBUF             1 // MANDATORY FOR ESP8266 BLOBS !! 
+#define LWIP_NETIF_TX_SINGLE_PBUF             1 // 1 is STRONGLY SUGGESTED FOR ESP8266 BLOBS !!
 #endif /* LWIP_NETIF_TX_SINGLE_PBUF */
 
 /**
@@ -2990,7 +2990,7 @@
 // so we do not define it. sntp server can come from dhcp server, or by
 // user.
 //#define SNTP_SERVER_ADDRESS	"pool.ntp.org"			// default
-#define SNTP_GET_SERVERS_FROM_DHCP	1
+#define SNTP_GET_SERVERS_FROM_DHCP	3
 #define SNTP_SET_SYSTEM_TIME_US(t,us)	do { struct timeval tv = { t, us }; settimeofday(&tv, NULL); } while (0)
 
 /*