adding posix port to cmake

CMakeLists.txt

@@ -15,7 +15,33 @@ target_sources(
   FreeRTOS/FreeRTOS-Kernel/timers.c
 )
 
-target_include_directories(${LIB_FREERTOS_NAME}
-    PUBLIC
-    FreeRTOS/FreeRTOS-Kernel/include
-)
+target_include_directories(
+  ${LIB_FREERTOS_NAME}
+  PUBLIC
+  FreeRTOS/FreeRTOS-Kernel/include
+)
+
+if(NOT FreeRTOS_PORT)
+  message(
+    STATUS
+      "FreeRTOS_PORT is empty. either set it or add port files manually."
+  )
+else()
+  if(FreeRTOS_PORT MATCHES "posix")
+    target_sources(
+      ${LIB_FREERTOS_NAME}
+      PRIVATE
+      FreeRTOS/FreeRTOS-Kernel/portable/ThirdParty/GCC/Posix/port.c
+    )
+    target_include_directories(
+      ${LIB_FREERTOS_NAME}
+      PRIVATE
+      FreeRTOS/FreeRTOS-Kernel/portable/ThirdParty/GCC/Posix/
+    )
+  else()
+    message(
+      ERROR
+        "Unsupportet FreeRTOS_PORT."
+    )
+  endif()
+endif()