First get the most recent u-boot code with:
git clone https://github.com/jwrdegoede/u-boot-sunxi.git
cd u-boot-sunxi
git checkout sunxi-next
compile the code:
CROSS_COMPILE=arm-linux-gnueabi- make Cubieboard2
and now flash it, where /dev/sdc is your SD card device:
sudo dd if=spl/sunxi-spl.bin of=/dev/sdc bs=1024 seek=8
sudo dd if=u-boot.img of=/dev/sdc bs=1024 seek=40