bbr start called after seq number updated. (ARMmbed#1875)

Author: deepakvenugopal

source/6LoWPAN/Thread/thread_extension_bbr.c

@@ -1441,14 +1441,17 @@ int thread_extension_bbr_address_set(int8_t interface_id, const uint8_t *addr_pt
 int thread_extension_bbr_sequence_number_set(int8_t interface_id, uint8_t seq_number)
 {
     thread_pbbr_t *this = thread_bbr_find_by_interface(interface_id);
-
     if (!this) {
         return -1;
     }
     if (seq_number) {
         this->sequence_number = --seq_number;
     }
-
+    if(this->backbone_interface_id < 0 || !this->pbbr_started) {
+        // it is ok to change seq number before the start of pbbr
+        return 0;
+    }
+    thread_extension_bbr_pbbr_start(this);
     return 0;
 }