[crypto] adding HMAC Key Derivation Function (HKDF) using SHA-256 (#4440)

This commit adds a new class `Crypto::HkdfSha256` which implements
HMAC-based Extract-and-Expand Key Derivation Function from RFC-5869
using SHA-256. It also adds a unit test for the new module.

tests/unit/test_util.cpp

@@ -30,7 +30,7 @@
 
 #include <ctype.h>
 
-void DumpBuffer(const char *aTextMessage, uint8_t *aBuffer, uint16_t aBufferLength)
+void DumpBuffer(const char *aTextMessage, const uint8_t *aBuffer, uint16_t aBufferLength)
 {
     enum
     {