Hello Dale Chin,
Regarding building LSDK image in Debian host, please refer to section "Emulate Ubuntu 18.04 environment using Docker container (optional)" in LSDK 1906 user manual, which could be downloaded from Layerscape SDK | NXP.
Please run the following command to generate Debian rootfs filesystem.
$ flex-builder -i mkrfs -r debian -a arm64
Thanks,
Yiping