update to lwIP-2.1.0: partial SACK support by default (de-selectable in menu) (#5126)

* update to lwIP-2.1.0rc1: partial SACK support
fix #4176

* hash fix

* get some flash back due to mistake in conf (fragmentation & reassembly was incorrectly enabled)
(ahah I scared you)

* add missing include files

* update to lwip-2.1.0(release) + remove unused lwIP's include files

* lwIP release 2.1.0, SACK is now default, bigger, no-SACK is selectable

* fix ldscript

* pio

* rename 'sack' option to 'feat'ure option, + IP fragmentation/reassembly

* merge, fix pio

* change internal/hidden string

* pio: more lwip2 configuration: + without sack for no change in flash footprint

tools/sdk/lwip2/include/lwip/apps/mdns_priv.h

@@ -40,6 +40,10 @@
 #include "lwip/apps/mdns_opts.h"
 #include "lwip/pbuf.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #if LWIP_MDNS_RESPONDER
 
 /* Domain struct and methods - visible for unit tests */
@@ -63,4 +67,8 @@ u16_t mdns_compress_domain(struct pbuf *pbuf, u16_t *offset, struct mdns_domain
 
 #endif /* LWIP_MDNS_RESPONDER */
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* LWIP_HDR_MDNS_PRIV_H */