tbbr: Add build flag HASH_ALG to let the user to select the SHA

The flag support the following values:
    - sha256 (default)
    - sha384
    - sha512

Change-Id: I7a49d858c361e993949cf6ada0a86575c3291066
Signed-off-by: Qixiang Xu <qixiang.xu@arm.com>
diff --git a/drivers/auth/mbedtls/mbedtls_crypto.c b/drivers/auth/mbedtls/mbedtls_crypto.c
index d8810d6..bc9ed3a 100644
--- a/drivers/auth/mbedtls/mbedtls_crypto.c
+++ b/drivers/auth/mbedtls/mbedtls_crypto.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2015-2017, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -7,6 +7,7 @@
 #include <crypto_mod.h>
 #include <debug.h>
 #include <mbedtls_common.h>
+#include <mbedtls_config.h>
 #include <stddef.h>
 #include <string.h>