Address PR cpomments reviews

1) move the change into Features from Changes, in the changLog
2) Change the feature alternative configuration MBEDTLS_ECDH_ALT
definition to function alternative defintions
MBEDTLS_ECDH_COMPUTE_SHARED_ALT and MBEDTLS_ECDH_GEN_PUBLIC_ALT
diff --git a/library/ecdh.c b/library/ecdh.c
index b66cb58..61380b6 100644
--- a/library/ecdh.c
+++ b/library/ecdh.c
@@ -38,7 +38,7 @@
 
 #include <string.h>
 
-#if !defined(MBEDTLS_ECDH_ALT)
+#if !defined(MBEDTLS_ECDH_GEN_PUBLIC_ALT)
 /*
  * Generate public key: simple wrapper around mbedtls_ecp_gen_keypair
  */
@@ -48,7 +48,9 @@
 {
     return mbedtls_ecp_gen_keypair( grp, d, Q, f_rng, p_rng );
 }
+#endif /* MBEDTLS_ECDH_GEN_PUBLIC_ALT */
 
+#if !defined(MBEDTLS_ECDH_COMPUTE_SHARED_ALT)
 /*
  * Compute shared secret (SEC1 3.3.1)
  */
@@ -82,7 +84,8 @@
 
     return( ret );
 }
-#endif /* MBEDTLS_ECDH_ALT */
+#endif /* MBEDTLS_ECDH_COMPUTE_SHARED_ALT */
+
 /*
  * Initialize context
  */