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/prot/ip.h

@@ -39,6 +39,10 @@
 
 #include "lwip/arch.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #define IP_PROTO_ICMP    1
 #define IP_PROTO_IGMP    2
 #define IP_PROTO_UDP     17
@@ -48,4 +52,8 @@
 /** This operates on a void* by loading the first byte */
 #define IP_HDR_GET_VERSION(ptr)   ((*(u8_t*)(ptr)) >> 4)
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* LWIP_HDR_PROT_IP_H */