[espidf] Warn with the expected path when the build yields no sdkconfig

esphome/espidf/bootloader.py

@@ -396,7 +396,10 @@ def _install_cached(app_config: dict, compiler: str, verbose: bool) -> Path | No
         if _build_standalone(tmp, verbose) != 0:
             return None
         if (built_config := toolchain._load_sdkconfig_json(tmp)) is None:
-            _LOGGER.debug("Bootloader build produced no sdkconfig.json")
+            # After a successful build this most likely means an IDF change.
+            _LOGGER.warning(
+                "Bootloader build produced no %s", tmp / "config" / "sdkconfig.json"
+            )
             return None
         # Only a build proven byte identical to a second one in a different
         # dir may be cached; timestamps or randomized signatures, present or

tests/unit_tests/test_espidf_bootloader.py

@@ -563,7 +563,9 @@ def test_ensure_returns_build_failure(tmp_path: Path) -> None:
     assert not list(root.glob(".build-*"))
 
 
-def test_ensure_soft_fails_when_build_yields_no_config(tmp_path: Path) -> None:
+def test_ensure_soft_fails_when_build_yields_no_config(
+    tmp_path: Path, caplog: pytest.LogCaptureFixture
+) -> None:
     def build_without_config(build_dir: Path, verbose: bool) -> int:
         build_dir.mkdir(parents=True, exist_ok=True)
         return 0
@@ -571,8 +573,10 @@ def test_ensure_soft_fails_when_build_yields_no_config(tmp_path: Path) -> None:
     with (
         _orchestration_env(tmp_path),
         patch.object(bootloader, "_build_standalone", side_effect=build_without_config),
+        caplog.at_level("WARNING"),
     ):
         assert bootloader.ensure_cached_bootloader() is False
+    assert "produced no" in caplog.text  # names the expected path
 
 
 @pytest.mark.parametrize("damage", ["missing-elf", "corrupt-bin"])