Added disabling of NA for Wi-SUN BR PPP backbone

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

source/6LoWPAN/ws/ws_bbr_api.c

@@ -239,6 +239,10 @@ static void wisun_bbr_na_send(int8_t interface_id, const uint8_t target[static 1
     if (!cur) {
         return;
     }
+    // Send NA only if it is enabled for the backhaul
+    if (!cur->send_na) {
+        return;
+    }
 
     buffer_t *buffer = icmpv6_build_na(cur, false, true, true, target, NULL, ADDR_UNSPECIFIED);
     protocol_push(buffer);

source/NWK_INTERFACE/Include/protocol.h

@@ -410,6 +410,7 @@ struct protocol_interface_info_entry {
     bool mpl_treat_realm_domains_as_one: 1;
     bool mpl_auto_domain_on_group_join: 1;
 #endif
+    bool send_na : 1;
     /* RFC 4861 Router Variables */
     bool ip_forwarding : 1;
     bool ip_multicast_forwarding : 1;

source/NWK_INTERFACE/protocol_core.c

@@ -485,6 +485,7 @@ static void protocol_core_base_finish_init(protocol_interface_info_entry_t *entr
     entry->dad_failures = 0;
     entry->icmp_tokens = 10;
     entry->mle_link_reject_tokens = 2;
+    entry->send_na = true; /* Default to on for now... */
     entry->ip_forwarding = true; /* Default to on for now... */
     entry->ip_multicast_forwarding = true; /* Default to on for now... */
 #ifdef HAVE_IPV6_ND
@@ -936,6 +937,7 @@ protocol_interface_info_entry_t *protocol_stack_interface_generate_ppp(eth_mac_a
     memcpy(new_entry->iid_eui64, iid64, 8);
     new_entry->send_mld = false;                 // No mld for PPP
     new_entry->dup_addr_detect_transmits = 0;    // No duplicate detection for PPP
+    new_entry->send_na = false;                  // No neighbor advertisements for PPP
 
     ns_list_add_to_start(&protocol_interface_info_list, new_entry);