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/tbbr/tbbr_cot.c b/drivers/auth/tbbr/tbbr_cot.c
index 4aaab39..01d6fb5 100644
--- a/drivers/auth/tbbr/tbbr_cot.c
+++ b/drivers/auth/tbbr/tbbr_cot.c
@@ -19,7 +19,7 @@
  * Maximum key and hash sizes (in DER format)
  */
 #define PK_DER_LEN			294
-#define HASH_DER_LEN			51
+#define HASH_DER_LEN			83
 
 /*
  * The platform must allocate buffers to store the authentication parameters