the kernel is opensource for sure
and the compilation process is
cp ./arch/arm/configs/clockworkpi_cpi3_defconfig .config
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf-
mkimage -A arm -O linux -T kernel -C none -a 0x40008000 -e 0x40008000 -n "Linux kernel" -d arch/arm/boot/zImage uImage
the configs will be there after you applied the patches ,gameshell offers the kernel patches for the mainline kernel