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/include/drivers/auth/mbedtls/mbedtls_config.h b/include/drivers/auth/mbedtls/mbedtls_config.h
index 96587ac..f8f2608 100644
--- a/include/drivers/auth/mbedtls/mbedtls_config.h
+++ b/include/drivers/auth/mbedtls/mbedtls_config.h
@@ -14,6 +14,13 @@
 #define TF_MBEDTLS_RSA_AND_ECDSA	3
 
 /*
+ * Hash algorithms currently supported on mbed TLS libraries
+ */
+#define TF_MBEDTLS_SHA256		1
+#define TF_MBEDTLS_SHA384		2
+#define TF_MBEDTLS_SHA512		3
+
+/*
  * Configuration file to build mbed TLS with the required features for
  * Trusted Boot
  */
@@ -66,6 +73,9 @@
 #endif
 
 #define MBEDTLS_SHA256_C
+#if (TF_MBEDTLS_HASH_ALG_ID != TF_MBEDTLS_SHA256)
+#define MBEDTLS_SHA512_C
+#endif
 
 #define MBEDTLS_VERSION_C