cmake: Leverage FreeRTOS's CMake Support

FreeRTOS-Kernel added CMake support in cdd406a62f31f. This commit
utilizes the CMake support for the demo app.

Note that CMAKE_CURRENT_SOURCE_DIR is specified for freertos_config as
SYSTEM because I want to ensure that all source and header files find
FreeRTOSConfig.h.

It still targets the POSIX port. If you intend to build it for another
PORT, you will need a toolchain file for your target (and probably a libc
to go along with it).

Ref:
 - FreeRTOS/FreeRTOS-Kernel#421
 - https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html

Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>

Author: yashi

CMakeLists.txt

@@ -1,20 +1,26 @@
 cmake_minimum_required(VERSION 3.20)
 project(libcsp-freertos)
 
+if(NOT DEFINED FREERTOS_PORT)
+  message(STATUS "Auto selecting GCC_POSIX for FREERTOS_PORT")
+  set(FREERTOS_PORT GCC_POSIX)
+endif()
+
 add_executable(demo)
 target_sources(demo PRIVATE
   main.c
   hooks.c
-  freertos/list.c
-  freertos/queue.c
-  freertos/tasks.c
-  freertos/timers.c
-  freertos/portable/ThirdParty/GCC/Posix/port.c
-  freertos/portable/ThirdParty/GCC/Posix/utils/wait_for_event.c)
-target_include_directories(demo PRIVATE
-  .
-  freertos/include
-  freertos/portable/ThirdParty/GCC/Posix)
-target_link_libraries(demo PRIVATE csp)
+)
+
+add_library(freertos_config INTERFACE)
+target_include_directories(freertos_config SYSTEM
+  INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})
 
+add_subdirectory(freertos)
 add_subdirectory(subprojects/libcsp)
+
+target_link_libraries(demo PRIVATE
+  freertos_kernel
+  freertos_config
+  csp
+)