Perform merges when substituting dict keys (#3062)

esphome/config_helpers.py

@@ -23,3 +23,23 @@ def read_config_file(path):
         return data["content"]
 
     return read_file(path)
+
+
+def merge_config(full_old, full_new):
+    def merge(old, new):
+        # pylint: disable=no-else-return
+        if isinstance(new, dict):
+            if not isinstance(old, dict):
+                return new
+            res = old.copy()
+            for k, v in new.items():
+                res[k] = merge(old[k], v) if k in old else v
+            return res
+        elif isinstance(new, list):
+            if not isinstance(old, list):
+                return new
+            return old + new
+
+        return new
+
+    return merge(full_old, full_new)