Merge remote-tracking branch 'origin/pr/2364' into mbedtls-2.16

* origin/pr/2364:
  Increase okm_hex buffer to contain null character
  Minor modifications to hkdf test
  Add explanation for okm_string size
  Update ChangeLog
  Reduce buffer size of okm
  Reduce Stack usage of hkdf test function
diff --git a/ChangeLog b/ChangeLog
index a0c2844..428a917 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -23,6 +23,7 @@
      Christian Walther in #2239.
    * Fix potential memory leak in X.509 self test. Found and fixed by
      Junhwan Park, #2106.
+   * Reduce stack usage of hkdf tests. Fixes #2195.
 
 Changes
    * Return from various debugging routines immediately if the