lowpan6.c: implement software CRC (e.g. required for use with ZEP)

src/include/netif/lowpan6.h

@@ -73,6 +73,8 @@ err_t lowpan6_if_init(struct netif *netif);
 /* pan_id in network byte order. */
 err_t lowpan6_set_pan_id(u16_t pan_id);
 
+u16_t lowpan6_calc_crc(const void *buf, u16_t len);
+
 #if !NO_SYS
 err_t tcpip_6lowpan_input(struct pbuf *p, struct netif *inp);
 #endif /* !NO_SYS */

src/include/netif/lowpan6_opts.h

@@ -60,7 +60,11 @@
 #endif
 
 #ifndef LWIP_6LOWPAN_HW_CRC
-#define LWIP_6LOWPAN_HW_CRC              1
+#define LWIP_6LOWPAN_HW_CRC              0
+#endif
+
+#ifndef LWIP_6LOWPAN_CALC_CRC
+#define LWIP_6LOWPAN_CALC_CRC(buf, len)  lowpan6_calc_crc(buf, len)
 #endif
 
 #ifndef LOWPAN6_DEBUG