bootloader: allow skip image validation on C2

BOOTLOADER_SKIP_VALIDATE_IN_DEEP_SLEEP is not supported on C2 due to
no RTC memory, but BOOTLOADER_SKIP_VALIDATE_ALWAYS should still be
supported.

examples/system/startup_time/pytest_startup_time.py

@@ -0,0 +1,20 @@
+# SPDX-FileCopyrightText: 2022 Espressif Systems (Shanghai) CO LTD
+# SPDX-License-Identifier: CC0-1.0
+import logging
+
+import pytest
+from pytest_embedded import Dut
+
+
+@pytest.mark.supported_targets
+@pytest.mark.generic
+@pytest.mark.parametrize('config', [
+    'defaults',
+    'always_skip',
+], indirect=True)
+def test_startup_time_example(dut: Dut) -> None:
+
+    res = dut.expect(r'\((\d+)\) [^:]+: App started!')
+    time = int(res[1])
+
+    logging.info(f'[Performance][startup_time]: {time}')