feat(freertos): Added FreeRTOS POSIX/Linux Simulator

* Added port layer from the FreeRTOS POSIX port, added
  additional port code for ESP-IDF.
* Created another hello world example using that POSIX
  port in tools/test_apps.
* Removed old linux app

tools/test_apps/linux_compatible/hello_world_linux_compatible/CMakeLists.txt

@@ -0,0 +1,7 @@
+# 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 3.16)
+
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+set(COMPONENTS main)
+project(hello_world)

tools/test_apps/linux_compatible/hello_world_linux_compatible/README.md

@@ -0,0 +1,45 @@
+| Supported Targets | ESP32 | ESP32-C2 | ESP32-C3 | ESP32-S2 | ESP32-S3 | Linux |
+| ----------------- | ----- | -------- | -------- | -------- | -------- | ----- |
+
+# Hello World Example Compatible with POSIX-port
+
+This is a version of the "Hello World" example compatible with the linux target. Just by using `idf.py (--preview) set-target <target>`, it can be compiled for chip targets as well as for the [FreeRTOS POSIX/Linux simulator](https://www.freertos.org/FreeRTOS-simulator-for-Linux.html), i.e., for running it on Linux. The applications can then be run on the chosen target.
+
+## Requirements
+
+If you want to use this example on Linux, you need a Linux machine as host. The remaining requirements are the same requirements as for [Unit Testing on Linux (using cmock)](https://docs.espressif.com/projects/esp-idf/en/stable/esp32/api-guides/linux-host-testing.html#requirements), except you do not need Ruby.
+
+## How to use example
+
+### Configure the project
+
+No special configuration is required, we also do not recommend changing configuration when compiled for the POSIX/Linux simulator as this is still in preview. If you have to configure something, use the usual IDF menuconfig:
+```
+idf.py menuconfig
+```
+
+### Build and Flash
+
+You can compile this example for chip targets, e.g. ESP32 and then run it by using:
+```
+idf.py set-target esp32
+idf.py build
+idf.py -p <port> flash monitor
+```
+
+If you want to build this example for the linux target and run it, use the same commands except setting the linux target and omitting the flash command:
+```
+idf.py --preview set-target linux
+idf.by build
+idf.py monitor
+```
+The linux target is still in preview, hence the necessary `--preview` argument. Flashing can be omitted on Linux.
+
+
+## Example folder contents
+
+The files in this project have the same structure as the files in the [original Hello World application](../../../../examples/get-started/hello_world/).
+
+## Example Output
+
+The output is similar to the output of the [original Hello World application](../../../../examples/get-started/hello_world/), except that no chip information is printed and there won't be any bootloader output on the linux target.

tools/test_apps/linux_compatible/hello_world_linux_compatible/main/CMakeLists.txt

@@ -0,0 +1,4 @@
+idf_component_register(SRCS "hello_world_main.c"
+                    INCLUDE_DIRS "")
+
+target_compile_options(${COMPONENT_LIB} PRIVATE "-Wno-format")

tools/test_apps/linux_compatible/hello_world_linux_compatible/main/hello_world_main.c

@@ -0,0 +1,24 @@
+/*
+ * SPDX-FileCopyrightText: 2010-2022 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: CC0-1.0
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include "sdkconfig.h"
+#include "freertos/FreeRTOS.h"
+#include "freertos/task.h"
+
+void app_main(void)
+{
+    printf("Hello world!\n");
+
+    for (int i = 10; i >= 0; i--) {
+        printf("Restarting in %d seconds...\n", i);
+        vTaskDelay(1000 / portTICK_PERIOD_MS);
+    }
+    printf("Restarting now.\n");
+    fflush(stdout);
+    exit(0);
+}

tools/test_apps/linux_compatible/hello_world_linux_compatible/pytest_hello_world_linux_compatible.py

@@ -0,0 +1,15 @@
+# SPDX-FileCopyrightText: 2022 Espressif Systems (Shanghai) CO LTD
+# SPDX-License-Identifier: CC0-1.0
+
+import pytest
+from pytest_embedded_idf.dut import IdfDut
+
+# Note that support for Linux target console applications hasn't been implemented for pytest-embedded yet
+# (https://github.com/espressif/pytest-embedded/issues/106)
+
+
+@pytest.mark.esp32
+@pytest.mark.esp32c3
+@pytest.mark.generic
+def test_hello_world_linux_compatible(dut: IdfDut) -> None:
+    dut.expect('Hello world!')