hardware_gen: Use logging.warning for import error

Updated YAML validation error message to use logging.warning,
forwarding the output to stderr. This would otherwise end up
in the CMake dts compatibility string.

tools/hardware_gen.py

@@ -24,7 +24,7 @@ from jinja2 import Environment, BaseLoader
 try:
     from jsonschema import validate
 except ImportError:
-    print("Skipping hardware YAML validation, `pip install jsonschema` to validate")
+    logging.warning("Skipping hardware YAML validation, `pip install jsonschema` to validate")
     def validate(*args, **kwargs):
         pass