all: rename Tlv namespace to tlv for consistency

This commit renames the Tlv namespace defined in
tlv.hpp to tlv to be more consistent with tlv::security
and tlv::nfd. A Tlv namespace alias is provided for
backwards compatibility.

Change-Id: I21278d6a077afe7776802c3296997d4c1c44c6c6
diff --git a/src/security/sec-tpm-osx.cpp b/src/security/sec-tpm-osx.cpp
index 3064d84..9f722fd 100644
--- a/src/security/sec-tpm-osx.cpp
+++ b/src/security/sec-tpm-osx.cpp
@@ -868,7 +868,7 @@
   if (signature.get() == 0)
     throw Error("Signature is NULL!\n");
 
-  return Block(Tlv::SignatureValue,
+  return Block(tlv::SignatureValue,
                make_shared<Buffer>(CFDataGetBytePtr(signature.get()),
                                    CFDataGetLength(signature.get())));
 }