bzImage is the minimal system generated by buildroot. It's max size is about 5MB.
build_rom is the script which removes the coreboot default payload and replaces it by the bzImage.
cbfstool is the coreboot utility to manipulate the coreboot rom image.
To flash the obtained coreboot.rom, use flashrom :
flashrom -p internal -w apu3_v4.8.0.4.rom
To build the minimal system :
git clone https://github.com/Keyyo-Private/buildroot_apu3.git
cd buildroot_apu3
make apu3_defconfig
make
The kernel is output/images/bzImage