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
=>bootm 0xa0000000#lx2160ardb
Thanks,
Yiping