buildroot/board/pc/genimage-bios.cfg

image disk.img {

  hdimage {
  }

  partition boot {
    in-partition-table = "no"
    image = "boot.img"
    offset = 0
    size = 512
  }

  partition grub {
    in-partition-table = "no"
    image = "grub.img"
    offset = 512
  }

  partition root {
    partition-type = 0x83
    image = "rootfs.ext2"
  }

}