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/security/security-common.hpp b/src/security/security-common.hpp
index 542e169..bd709f5 100644
--- a/src/security/security-common.hpp
+++ b/src/security/security-common.hpp
@@ -24,6 +24,8 @@
 
 #include "../common.hpp"
 
+#define NDN_CXX_KEEP_SECURITY_V1_ALIASES
+
 namespace ndn {
 
 namespace signed_interest {