Whether NXP supports compile uboot only, without compile kernel.
You could download the latest LSDK 21.08 from the following link.
https://www.nxp.com/design/software/embedded-software/linux-software-and-development-tools/layerscap...
Then build u-boot with the following command
$ source setup.env
$ flex-builder -c atf -b xspi -m lx2160ardb_rev2
Build atf command with the following command
$ flex-builder -c atf -m lx2160ardb_rev2
