GRUB2 image creation tool (shell-script version)

The script generates disk image prepared for two partitions but includes
only the first partition, which contains files for GRUB2. All image
preparation steps that need superuser privileges are conducted by this
script. The final image is just an concatenation of grub2-head.img and
the boot scenario file-system image (which will be generated with
tool/rump). The final step of writing the entire image to a disk must be
executed later by

  sudo dd if=<image file> of=<device> bs=8M conv=fsync

The commit also includes the prepared image grub2-head.img.