encoding: Improving structure and documentation of block helpers

This commit add several new helpers to simplify operations with
std::string:

- prependStringBlock
- makeStringBlock
- readString

The following functions are deprecated and their replacements:

- nonNegativeIntegerBlock (use makeNonNegativeIntegerBlock)
- prependBooleanBlock (use prependEmptyBlock)
- booleanBlock (use makeEmptyBlock)
- dataBlock (use makeBinaryBlock)
- nestedBlock (use makeNestedBlock)

Change-Id: Ic595ae64fc9c80c2c04e5fde1d8812e8b9debd60
Refs: #2951
diff --git a/tests/unit-tests/data.t.cpp b/tests/unit-tests/data.t.cpp
index 262f2ac..ab9cee6 100644
--- a/tests/unit-tests/data.t.cpp
+++ b/tests/unit-tests/data.t.cpp
@@ -194,7 +194,7 @@
 
   b = SignatureSha256WithRsa();
   static const uint8_t someData[256] = {};
-  Block signatureValue = dataBlock(tlv::SignatureValue, someData, sizeof(someData));
+  Block signatureValue = makeBinaryBlock(tlv::SignatureValue, someData, sizeof(someData));
   b.setValue(signatureValue);
   BOOST_CHECK_EQUAL(a == b, false);
   BOOST_CHECK_EQUAL(a != b, true);
@@ -382,8 +382,7 @@
   Block signatureInfo(tlv::SignatureInfo);
   // SignatureType
   {
-    signatureInfo.push_back
-      (nonNegativeIntegerBlock(tlv::SignatureType, Signature::Sha256WithRsa));
+    signatureInfo.push_back(makeNonNegativeIntegerBlock(tlv::SignatureType, Signature::Sha256WithRsa));
   }
   // KeyLocator
   {