This is supported only for buildroot root filesystems. You can instruct flex-builder to create a buildroot rootfs by specifying -r option:
bpe_1-1600748384498.png
and configuring the options in the corresponding file under configs/buildroot/ as appropriate:
bpe_0-1600748133442.png
More details can be found in docs/flexbuild_usage.txt
Hope this helps
Platon