examples: Add example for fastest startup time

Example includes README and sdkconfig.defaults with notes about trade-offs
made for minimum boot time.

examples/system/startup_time/example_test.py

@@ -0,0 +1,30 @@
+from __future__ import print_function
+
+import re
+
+import ttfw_idf
+
+
+@ttfw_idf.idf_example_test(env_tag='Example_GENERIC', target=['esp32', 'esp32s2', 'esp32c3'])
+def test_startup_time_example(env, extra_data):
+    app_name = 'startup_time'
+    dut = env.get_dut(app_name, 'examples/system/startup_time')
+    dut.start_app()
+
+    res = dut.expect(re.compile(r'\((\d+)\) [^:]+: App started!'))
+    time = int(res[0])
+
+    # Allow ci-dashboard to track startup times
+    print('------ startup time info ------\n'
+          '[app_name] {}\n'
+          '[startup_time] {}\n'
+          '[config] {}\n'
+          '[target] {}\n'
+          '------ startup time end ------'.format(app_name,
+                                                  time,
+                                                  dut.app.config_name,
+                                                  dut.TARGET))
+
+
+if __name__ == '__main__':
+    test_startup_time_example()