ci: add build test for custom console UART pins

tools/test_apps/system/build_test/CMakeLists.txt

@@ -0,0 +1,6 @@
+# The following lines of boilerplate have to be in your project's
+# CMakeLists in this exact order for cmake to work correctly
+cmake_minimum_required(VERSION 3.5)
+
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+project(test_build)

tools/test_apps/system/build_test/README.txt

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+This project is for testing if the application can be built with a particular sdkconfig setting.
+
+To add new configuration, create one more sdkconfig.ci.NAME file in this directory.
+To make the configuration target-specific, add a CONFIG_IDF_TARGET="name" line.
+
+If you need to test for anything other than building, create another test project.

tools/test_apps/system/build_test/main/CMakeLists.txt

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+idf_component_register(SRCS "test_main.c"
+                       INCLUDE_DIRS ".")

tools/test_apps/system/build_test/main/test_main.c

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+void app_main(void)
+{
+}

tools/test_apps/system/build_test/sdkconfig.ci.custom_uart

@@ -0,0 +1 @@
+CONFIG_ESP_CONSOLE_UART_CUSTOM=y