PineCube is supported in Buildroot since version 2023.11. See the file board/pine64/pinecube/readme.txt in the Buildroot repository for details how to build it.

There is also available Buildroot fork by Elimo Engineering, you can find the repository on Elimo’s GitHub account and build instructions in the board support directory readme.

The most important thing that this provides is support for the S3’s DDR3 in u-boot. Unfortunately mainline u-boot does not have that yet, but the u-boot patches from Daniel Fullmer’s NixOS repo were easy enough to use on buildroot.

This should get you a functional system that boots to a console on UART0. It’s pretty fast too, getting there in 1.5 seconds from u-boot to login prompt.