In SDK 2016.2 you had the possibility of using Xilflash and Xilisf with FreeRTOS, in version 2016.3 this is no longer available.
Is this a mistake from Xilinx? After all, you can cause a lot of threading problems anyhow.
Why not allow it and write a warning?
To workaround this, you can make a local copy of the library (into mylibs/sw_services/xilflash_v4_2
Open the mld in the data folder and update the REQUIRES_OS
OPTION REQUIRES_OS = (standalone xilkernel freertos823_xilinx);
Then in SDK, point to the mylibs and the updated version od the xilflash should be picked up