Ability to force the Entropy module to use SHA-256 as its basis

By default the SHA-512 module is used if both are available. On some
systems, SHA-256 is the better choice.

Contributed by: Gergely Budai
diff --git a/ChangeLog b/ChangeLog
index baf67fe..2e2ecb8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -11,6 +11,8 @@
      and CRLs
    * Single Platform compatilibity layer (for memory / printf / fprintf)
    * Ability to provide alternate timing implementation
+   * Ability to force the entropy module to use SHA-256 as its basis
+     (POLARSSL_ENTROPY_FORCE_SHA256)
 
 Changes
    * Deprecated the Memory layer