Support for building and running external apps (#112)

Libraries/elf_loader/README.md

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+## Description
+
+[![Component Registry](https://components.espressif.com/components/espressif/elf_loader/badge.svg)](https://components.espressif.com/components/espressif/elf_loader)
+
+Espressif ELF(Executable and Linkable Format) loader is a software development kit that is developed based on the ESP-IDF, mainly used to load ELF file compiled based on ESP32 series SoCs to the executable memory area, then link and execute it.
+
+In this way, the application does not need compile into the whole firmware in advance. It is like running the compiled program through terminal input `./main.o` on the Ubuntu platform automatically, which realizes the separation of application and kernel.
+
+This ELF loader supports following SoCs:
+
+- ESP32
+- ESP32-S2, support running ELF in PSRAM
+- ESP32-S3, support running ELF in PSRAM
+
+### Usage
+
+#### Firmware
+
+Add a dependency on this component in your component or project's idf_component.yml file.
+
+    ```yml
+    dependencies:
+      espressif/elf_loader: "0.*"
+    ```
+
+Enable ELF loader in the menuconfig:
+
+```
+Component config  --->
+    ESP-ELFLoader Configuration  --->
+        [*] Enable Espressif ELF Loader
+```
+
+Add API calls in your project as follows:
+
+```c
+#include "esp_elf.h"
+
+    esp_elf_t elf;
+
+    // Your Code
+
+    esp_elf_init(&elf);
+    esp_elf_relocate(&elf, elf_file_data_bytes);
+    esp_elf_request(&elf, 0, argc, argv);
+    esp_elf_deinit(&elf);
+```
+
+#### ELF APP
+
+To use this feature to compile ELF file, including the required CMake file in your project's CMakeLists.txt file after the line project(XXXX).
+
+```cmake
+project(XXXX)
+
+# Add
+include(elf_loader)
+project_elf(XXXX)
+```
+
+Build the project as an ordinary ESP-IDF project, and then the ELF file named `XXXX.app.elf` is in the build directory.
+
+### Adding the Component to Your Project
+
+Please use the component manager command add-dependency to add the elf_loader component as a dependency in your project. During the CMake step, the component will be downloaded automatically.
+
+```
+idf.py add-dependency "espressif/elf_loader=*"
+```
+
+### Examples
+
+Please use the component manager command create-project-from-example to create a project from the example template.
+
+```
+idf.py create-project-from-example "espressif/elf_loader=*:elf_loader_example"
+```
+
+This command will download the example elf_loader_example into the current folder. You can navigate into it to build and flash the example.
+
+Alternatively, you can download examples from the esp-iot-solution repository:
+1. [build_elf_file_example](https://github.com/espressif/esp-iot-solution/tree/master/examples/elf_loader/build_elf_file_example)
+2. [elf_loader_example](https://github.com/espressif/esp-iot-solution/tree/master/examples/elf_loader/elf_loader_example)