Added disabling of NA for Thread BR PPP backbone

If backbone is PPP and not ethernet, BR no longer sends neighbor
advertisements to the interface when new address is registered.

Author: Mika Leppänen

source/6LoWPAN/Thread/thread_bbr_api.c

@@ -992,6 +992,10 @@ int thread_bbr_na_send(int8_t interface_id, const uint8_t target[static 16])
     if (!cur) {
         return -1;
     }
+    // Send NA only if it is enabled for the backhaul
+    if (!cur->send_na) {
+        return -1;
+    }
 
     buffer_t *buffer = icmpv6_build_na(cur, false, true, true, target, NULL, ADDR_UNSPECIFIED);
     protocol_push(buffer);