Add FreeRTOS config directory to include dirs (#548)

This allows the application write to set FREERTOS_CONFIG_FILE_DIRECTORY
to whichever directory the FreeRTOSConfig.h file exists in.

This was reported here - https://github.com/FreeRTOS/FreeRTOS-Kernel/issues/545

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>

portable/ThirdParty/GCC/RP2040/library.cmake

@@ -27,7 +27,8 @@ target_sources(FreeRTOS-Kernel INTERFACE
 )
 
 target_include_directories(FreeRTOS-Kernel INTERFACE
-        ${CMAKE_CURRENT_LIST_DIR}/include)
+        ${CMAKE_CURRENT_LIST_DIR}/include
+        ${FREERTOS_CONFIG_FILE_DIRECTORY})
 
 target_link_libraries(FreeRTOS-Kernel INTERFACE
         FreeRTOS-Kernel-Core