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/src/common-pch.hpp b/src/common-pch.hpp
index 06e8912..ad4bbff 100644
--- a/src/common-pch.hpp
+++ b/src/common-pch.hpp
@@ -51,6 +51,6 @@
 #include <boost/system/error_code.hpp>
 
 // Other useful headers to precompile
-#include "security/cryptopp.hpp"
+#include "security/v1/cryptopp.hpp"
 
 #endif // NDN_COMMON_PCH_HPP