I want to use U-BOOT.
Is it available?

It seem only BLDR and ATF. (on UART)

I don’t think Archermind provide u-boot for this board. In fact u-boot for most of the other 96Boards has been developed by third parties. If you want to have a go at a port then starting with u-boot for Dragonboard 410c would be recommended (it is chain loaded by LittleKernel and something similar could be established on X20).