security: Reorganizing source code to prepare for support of two version of NDN certificates

This commit also removes unused ndn_digestSha256 function and deprecates
crypto::sha256 in favor of crypto::computeSha256Digest in util/crypto.hpp.

Change-Id: I24ee50ff073a96b868633bdf2cfade412d3605f3
Refs: #3098
diff --git a/tests/unit-tests/link.t.cpp b/tests/unit-tests/link.t.cpp
index 6071158..1627b3c 100644
--- a/tests/unit-tests/link.t.cpp
+++ b/tests/unit-tests/link.t.cpp
@@ -21,7 +21,7 @@
 
 #include "link.hpp"
 #include "security/key-chain.hpp"
-#include "security/cryptopp.hpp"
+#include "security/v1/cryptopp.hpp"
 #include "encoding/buffer-stream.hpp"
 
 #include "boost-test.hpp"