Please refer to the following commands.
$ flex-builder -i mkrfs -F
('-F' option can force build the target userland from scratch regardless of the FETCH_PREBUILT_LSDK_RFS=y configured in configs/sdk.yml)
$ flex-builder -c apps
(build application components against ubuntu main userland)
$ flex-builder -i merge-component
(merge app components to main userland)
$ flex-builder -i packrfs
(pack target userland as rootfs_lsdk2108_ubuntu_main_arm64.tgz)