u-boot and cssu-thumb are supposed to live happily together, please describe what your problem is.

just a hint - you need either kernel-power-flasher or kernel-cssu-flasher installed to have the correct dependencies for cssu-thumb. that means, that after installing u-boot, you SHOULD NOT uninstall -flasher packages. If you have kernel-power, then you need kernel-power-bootimg, if you have kernel-cssu, then you need kernel-cssu-bootimg respectively.