The only way to update u-boot I can recommend is to install the latest Android firmware from manufacturer. I haven't tried and I don't recommend flashing u-boot from a different device.S905 devices without freezes are WeTek Hub, WeTek Play2 and Odroid-C2. All my non-branded S905 devices "died" in one or another way, I can't check.
@kszaq.......I would be happy to donate to you a new Nexbox A95X S905 for you to play with if it helps resolve the freezing on boot issue. PVT me if I can help. Cheers.