Allow to overrid the check for p->ref==1 in TX packets (there *are* netif drivers that can handle this)

src/include/lwip/ip.h

@@ -54,6 +54,12 @@ extern "C" {
 #endif /* IP_HDRINCL */
 #define IP_HDRINCL  NULL
 
+/** pbufs passed to IP must have a ref-count of 1 as their payload pointer
+    gets altered as the packet is passed down the stack */
+#ifndef LWIP_IP_CHECK_PBUF_REF_COUNT_FOR_TX
+#define LWIP_IP_CHECK_PBUF_REF_COUNT_FOR_TX(p) LWIP_ASSERT("p->ref == 1", (p)->ref == 1)
+#endif
+
 #if LWIP_NETIF_HWADDRHINT
 #define IP_PCB_ADDRHINT ;u8_t addr_hint
 #else