ECDH alternative implementation support

Add alternative implementation support for ECDH at the higher layer
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index c4b8995..a29312a 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -238,6 +238,7 @@
 //#define MBEDTLS_BLOWFISH_ALT
 //#define MBEDTLS_CAMELLIA_ALT
 //#define MBEDTLS_DES_ALT
+//#define MBEDTLS_ECDH_ALT
 //#define MBEDTLS_XTEA_ALT
 //#define MBEDTLS_MD2_ALT
 //#define MBEDTLS_MD4_ALT