fixup! LWIP PBUF_POOL_BUFSIZE increased to fit also IPv6 header

Author: Veijo Pesonen

features/FEATURE_LWIP/lwip-interface/lwipopts.h

@@ -151,12 +151,14 @@
 #undef PBUF_POOL_BUFSIZE
 #define PBUF_POOL_BUFSIZE           MBED_CONF_LWIP_PBUF_POOL_BUFSIZE
 #else
+#ifndef PBUF_POOL_BUFSIZE
 #if LWIP_IPV6
 #define PBUF_POOL_BUFSIZE               LWIP_MEM_ALIGN_SIZE(TCP_MSS+20+40+PBUF_LINK_ENCAPSULATION_HLEN+PBUF_LINK_HLEN)
-#else
+#elif LWIP_IPV4
 #define PBUF_POOL_BUFSIZE               LWIP_MEM_ALIGN_SIZE(TCP_MSS+20+20+PBUF_LINK_ENCAPSULATION_HLEN+PBUF_LINK_HLEN)
 #endif
 #endif
+#endif
 
 #ifdef MBED_CONF_LWIP_MEM_SIZE
 #undef MEM_SIZE