[tools] Fixed infinite loop in config scan due to list comparison error

tools/config.py

@@ -444,7 +444,7 @@ def load_resources(self, resources):
             self.add_config_files(resources.json_files)
 
             # Add features while we find new ones
-            features = self.get_features()
+            features = set(self.get_features())
             if features == prev_features:
                 break
 

tools/test/config_test/test28/mbed_app.json

@@ -0,0 +1,17 @@
+{
+    "custom_targets": {
+        "test_target": {
+            "core": "Cortex-M0",
+            "extra_labels": [],
+            "features": [],
+            "default_build": "standard"
+        }
+    },
+    "target_overrides": {
+        "*": {
+            "target.features_add": ["UVISOR"],
+            "target.extra_labels_add": ["UVISOR_SUPPORTED"]
+        }
+    }
+}
+

tools/test/config_test/test28/test_data.py

@@ -0,0 +1,8 @@
+# Testing when adding two features
+
+expected_results = {
+    "test_target": {
+        "desc": "test uvisor feature",
+        "expected_features": ["UVISOR"]
+    }
+}