ECDH alternative implementation support

Add alternative implementation support for ECDH at the higher layer
diff --git a/ChangeLog b/ChangeLog
index 243bd6b..a2a2a36 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -36,6 +36,10 @@
    * Clarify ECDSA documentation and improve the sample code to avoid
      misunderstandings and potentially dangerous use of the API. Pointed out
      by Jean-Philippe Aumasson.
+   * Add support for alternative implementation for ECDH, controlled by new
+     configuration flag MBEDTLS_ECDH_ALT in config.h.
+     Alternative Ecdh is supported for implementation of `mbedtls_ecdh_gen_public`
+     and `mbedtls_ecdh_compute_shared`.
 
 = mbed TLS 2.5.0 branch released 2017-05-17