Add a example demonstrating hybrid boot. The main ingredients are:
- grub2 stage has `uefi` and `legacy` options
- GPT partition layout
- BIOS boot partition so that core image for grub2 legacy can
be stored (small, 1MB, type 21686148-6449-6E6F-744E-656564454649)
- ESP partition so UEFI can load grub*.efi (via the shim)