4. Buildroot

Get the newest version of Buildroot from here and unpack the archive.
Enter the Buildroot folder:
cd buildroot/configs/

Download the config file from my repository:
wget https://github.com/petit-miner/Blueberry-PI/raw/master/Software/blueberrypi_defconfig

leave the configs folder:
cd ..

Apply the config file:
make blueberrypi_defconfig

If you want to you can further customize your buildroot:
make menuconfig

After configuring buildroot, built it:
make
Lean back and grab a coffee, this will take a while.

After successfully building buildroot you can find it here:
output/images/rootfs.tar