examples: add import prebuilt example

examples/build_system/cmake/import_prebuilt/CMakeLists.txt

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+# 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(import_prebuilt)

examples/build_system/cmake/import_prebuilt/README.md

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+# Import Prebuilt Library Example
+
+This example illustrates how to import a prebuilt static library in the ESP-IDF build system.
+
+## Example Flow
+
+Users need to first build the project in the [prebuilt](prebuilt) subdirectory:
+
+```
+cd prebuilt
+idf.py build
+```
+
+This builds a component named [prebuilt](prebuilt/components/prebuilt), which has private dependency on ESP-IDF components `spi_flash`, `log` and `app_update` (see [its CMakeLists.txt](prebuilt/components/prebuilt/CMakeLists.txt)). Once built, the archive file `libprebuilt.a`, along with the header file `prebuilt.h`, is automatically copied to the [`main` component](main) of this example project. 
+
+The [`main` component's CMakeLists.txt](main/CMakeLists.txt) demonstrates how to import `libprebuilt.a` and link it to `main` so that the definitions inside can be used. 
+It also demonstrates how to specify the same dependencies the original component had so as to properly resolve symbols used inside the prebuilt library.
+
+Users can then return to this directory and build the main example:
+
+```
+cd ..
+idf.py build
+```
+
+
+### Output
+
+The example simply outputs the current running partition.
+
+```
+I (319) prebuilt: The running partition is 'factory'!
+```
+
+---
+
+There is a discussion on importing prebuilt libraries in the programming guide under `API Guides` -> `Build System` -> `Using Prebuilt Libraries with Components`

examples/build_system/cmake/import_prebuilt/main/CMakeLists.txt

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+idf_component_register(SRCS "main.c"
+                    INCLUDE_DIRS ".")
+
+# Import the library, specifying a target name and the library path.
+# The private dependencies of the library is also specified.
+add_prebuilt_library(prebuilt "libprebuilt.a"
+             PRIV_REQUIRES spi_flash app_update log)
+
+# `main` calls a function from the library, so link it to `main`
+target_link_libraries(${COMPONENT_LIB} PRIVATE prebuilt)
+
+

examples/build_system/cmake/import_prebuilt/main/main.c

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+/* Import Prebuilt Library Example
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+
+// Include the prebuilt library's header file so as to be able 
+// to reference `prebuilt_func` here.
+#include "prebuilt.h"
+
+void app_main(void)
+{
+    prebuilt_func();
+}

examples/build_system/cmake/import_prebuilt/main/project_include.cmake

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+# For users checking this example, ignore the following code. This is so that
+# the prebuilt project is built automatically in ESP-IDF CI.
+if("$ENV{CI}")
+    execute_process(COMMAND ${IDFTOOL} build
+                    WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}/prebuilt)
+endif()

examples/build_system/cmake/import_prebuilt/prebuilt/CMakeLists.txt

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+cmake_minimum_required(VERSION 3.5)
+
+# Since we're only interested in the prebuilt library, trim
+# the build
+set(COMPONENTS prebuilt main esptool_py)
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+
+project(prebuilt C)

examples/build_system/cmake/import_prebuilt/prebuilt/components/prebuilt/CMakeLists.txt

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+idf_component_register(SRCS prebuilt.c
+                       INCLUDE_DIRS "."
+                       PRIV_REQUIRES app_update spi_flash log)
+
+# After build, copy the archive file and header file to parent example directory's main component
+add_custom_command(TARGET ${COMPONENT_LIB}
+                  POST_BUILD
+                  COMMAND ${CMAKE_COMMAND} -E copy $<TARGET_FILE:${COMPONENT_LIB}> ${CMAKE_SOURCE_DIR}/../main
+                  COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_LIST_DIR}/prebuilt.h ${CMAKE_SOURCE_DIR}/../main
+                  COMMENT "Copying built archive file and header to parent example directory...")

examples/build_system/cmake/import_prebuilt/prebuilt/components/prebuilt/prebuilt.c

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+/* Import Prebuilt Library Example
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+#include "esp_ota_ops.h"
+#include "esp_partition.h"
+#include "esp_log.h"
+
+const char *TAG = "prebuilt";
+
+void prebuilt_func(void)
+{
+    const esp_partition_t* running_partition = esp_ota_get_running_partition();
+    ESP_LOGI(TAG, "The running partition is '%s'!", running_partition->label);
+}
+
+

examples/build_system/cmake/import_prebuilt/prebuilt/components/prebuilt/prebuilt.h

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+/* Import Prebuilt Library Example
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+#pragma once
+
+void prebuilt_func(void);

examples/build_system/cmake/import_prebuilt/prebuilt/main/CMakeLists.txt

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

examples/build_system/cmake/import_prebuilt/prebuilt/main/main.c

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+#include <stdio.h>
+
+void app_main(void)
+{
+    printf("Hello World!\n");
+}