You could boot tiny Linux on the target board, and connecting the target board to Ethernet, then execute the following commands.
flex-installer -i pf -d /dev/mmcblk0
$ wget https://www.nxp.com/lgfiles/sdk/lsdk2108/boot_LS_arm64_lts_5.10.tgz
$ wget https://www.nxp.com/lgfiles/sdk/lsdk2108/rootfs_lsdk2108_ubuntu_main_arm64.tgz
$ flex-installer -b boot_LS_arm64_lts_5.10.tgz
-r rootfs_lsdk2108_ubuntu_main_arm64.tgz -d /dev/mmcblk0
You also could run the above commands on the host PC.
$ wget https://www.nxp.com/lgfiles/sdk/lsdk2108/flex-installer && chmod +x flex-installer && sudo mv flex-installer /usr/bin
Please specify "-d" to emmc device name on the host PC.