feat(examples): add sdkconfig integration test examples (#1718)

Self-verifying examples that demonstrate two undocumented sdkconfig features:

- espidf-sdkconfig-defaults: SDKCONFIG_DEFAULTS layering via
  board_build.cmake_extra_args with multiple semicolon-separated files
- espidf-sdkconfig-custom-path: custom sdkconfig output path via
  board_build.esp-idf.sdkconfig_path

Both use _Static_assert to verify the configuration took effect at
compile time, making them self-checking integration tests.

Author: m-mcgowan

examples/espidf-sdkconfig-custom-path/.gitignore

@@ -0,0 +1 @@
+.pio/

examples/espidf-sdkconfig-custom-path/CMakeLists.txt

@@ -0,0 +1,3 @@
+cmake_minimum_required(VERSION 3.16)
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+project(sdkconfig_custom_path_test)

examples/espidf-sdkconfig-custom-path/platformio.ini

@@ -0,0 +1,14 @@
+; PlatformIO Project Configuration File
+;
+; This example demonstrates using a custom sdkconfig file path via
+; board_build.esp-idf.sdkconfig_path. Instead of the default
+; "sdkconfig.<env>" naming convention, a custom filename is used.
+;
+; sdkconfig.custom sets CONFIG_LOG_DEFAULT_LEVEL=4 (DEBUG)
+; The source code uses _Static_assert to verify the config was applied.
+
+[env:esp32dev]
+platform = espressif32
+framework = espidf
+board = esp32dev
+board_build.esp-idf.sdkconfig_path = sdkconfig.custom