Address PR review comments

1) update ChangLog to have new feature in Features instead of Changes
2) Change MBEDTLS_ECDSA_ALT to function specific alternative definitions:
MBEDTLS_ECDSA_SIGN_ALT, MBEDTLS_ECDSA_VERIFY_ALT and MBEDTLS_ECDSA_GENKEY_ALT
diff --git a/ChangeLog b/ChangeLog
index 94eba42..040632c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -36,10 +36,13 @@
    * 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 ECDSA, controlled by new
-     configuration flag MBEDTLS_ECDSA_ALT in config.h.
+
+Features
+   * Add support for alternative implementations for ECDSA, controlled by new
+     configuration flags MBEDTLS_ECDSA_SIGN_ALT, MBEDTLS_ECDSA_VERIFY_ALT and
+     MBEDTLS_ECDSDA_GENKEY_AT in config.h.
      The following functions from the ECDSA module can be replaced
-     with an alternative implementation:
+     with alternative implementation:
      mbedtls_ecdsa_sign(), mbedtls_ecdsa_verify() and mbedtls_ecdsa_genkey().
 
 = mbed TLS 2.5.0 branch released 2017-05-17