ECDSA alternative support

Support for alternative implementation of ECDSA, at the higher layer
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index c4b8995..54dc237 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_ECDSA_ALT
 //#define MBEDTLS_XTEA_ALT
 //#define MBEDTLS_MD2_ALT
 //#define MBEDTLS_MD4_ALT