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/include/mbedtls/config.h b/include/mbedtls/config.h
index 54dc237..7c06ec4 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -238,7 +238,6 @@
 //#define MBEDTLS_BLOWFISH_ALT
 //#define MBEDTLS_CAMELLIA_ALT
 //#define MBEDTLS_DES_ALT
-//#define MBEDTLS_ECDSA_ALT
 //#define MBEDTLS_XTEA_ALT
 //#define MBEDTLS_MD2_ALT
 //#define MBEDTLS_MD4_ALT
@@ -295,6 +294,9 @@
 //#define MBEDTLS_AES_SETKEY_DEC_ALT
 //#define MBEDTLS_AES_ENCRYPT_ALT
 //#define MBEDTLS_AES_DECRYPT_ALT
+//#define MBEDTLS_ECDSA_VERIFY_ALT
+//#define MBEDTLS_ECDSA_SIGN_ALT
+//#define MBEDTLS_ECDSA_GENKEY_ALT
 
 /**
  * \def MBEDTLS_ECP_INTERNAL_ALT