link: add Link abstraction

Change-Id: I1e4bafe8559ef87135de6cb6f209fc94dfa231a3
Refs: #2587
diff --git a/src/encoding/tlv.hpp b/src/encoding/tlv.hpp
index 01a8679..da7b8ea 100644
--- a/src/encoding/tlv.hpp
+++ b/src/encoding/tlv.hpp
@@ -84,6 +84,9 @@
   SignatureType = 27,
   KeyLocator    = 28,
   KeyDigest     = 29,
+  LinkPreference = 30,
+  LinkDelegation = 31,
+  SelectedDelegation = 32,
 
   AppPrivateBlock1 = 128,
   AppPrivateBlock2 = 32767