BPI-M3 want to have custom built uboot/kernel

I have a custom board based on BPI-M3, i would like to have a customized uboot and kernel based on Raspbian Pi. I have followed https://www.raspberrypi.org/documentation/linux/kernel/building.md to build the kernel (device tree structure) , but for uboot i am stuck. Any suggestions or links to any similar topics will help me a lot. Thanks