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/security/digest-sha256.t.cpp b/tests/unit-tests/security/digest-sha256.t.cpp
index f6e2eb0..67b6730 100644
--- a/tests/unit-tests/security/digest-sha256.t.cpp
+++ b/tests/unit-tests/security/digest-sha256.t.cpp
@@ -39,7 +39,7 @@
   using namespace CryptoPP;
 
   char content[6] = "1234\n";
-  ConstBufferPtr buf = crypto::sha256(reinterpret_cast<uint8_t*>(content), 5);
+  ConstBufferPtr buf = crypto::computeSha256Digest(reinterpret_cast<uint8_t*>(content), 5);
 
   BOOST_CHECK_EQUAL(SHA256_RESULT, toHex(buf->buf(), buf->size(), false));
 }