Use existing variable containing full path

Signed-off-by: Thomas Daubney <thomas.daubney@arm.com>
diff --git a/scripts/generate_driver_wrappers.py b/scripts/generate_driver_wrappers.py
index edd98a2..624ab81 100755
--- a/scripts/generate_driver_wrappers.py
+++ b/scripts/generate_driver_wrappers.py
@@ -181,7 +181,7 @@
     crypto_core_directory = build_tree.crypto_core_directory(project_root)
 
     output_directory = args.output_directory if args.output_directory is not None else \
-        os.path.join(project_root, crypto_core_directory)
+        crypto_core_directory
 
     template_directory = args.template_dir if args.template_dir is not None else \
         os.path.join(project_root,