Hello songhee yun,
In bootpartition tar ball, you could find tiny itb image lsdk2004_yocto_tiny_LS_arm64.itb, which includes Kernel image, dtb file and ramdisk rootfs filesystem.
You could boot up the system with the following commands
=> load mmc 0:2 0xa0000000 lsdk2004_yocto_tiny_LS_arm64.itb
You could consider to use Yocto 3.0 release from readme - qoriq-components/yocto-sdk - Repo manifest for QorIQ Yocto SDK
Using the following command Kernel image and ramdisk rootfs filesystem will be generated.
$./setup-env -m lx2160ardb
$ bitbake fsl-image-networking