Git clone:

git clone git://git.denx.de/u-boot.git

Configure:

cd u-boot
CROSS_COMPILE=arm-linux-gnueabi- make clean
CROSS_COMPILE=arm-linux-gnueabi- make Cubieboard2_defconfig

Compile:

CROSS_COMPILE=arm-linux-gnueabi- make -j6

Installation:

dd if=u-boot-sunxi-with-spl.bin of=/dev/sdX bs=1024 seek=8

Where /dev/sdX is the device file for your SD card. (Hint: use dmesg to find it out)