Use default value for the `filename` parameter

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
diff --git a/tf-psa-crypto/scripts/config.py b/tf-psa-crypto/scripts/config.py
index ef76d3b..8002392 100755
--- a/tf-psa-crypto/scripts/config.py
+++ b/tf-psa-crypto/scripts/config.py
@@ -134,7 +134,7 @@
     and modify the configuration.
     """
 
-    def __init__(self, filename):
+    def __init__(self, filename=None):
         """Read the PSA crypto configuration files."""
 
         super().__init__()