Step: `grub`
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Install the GRUB bootloader to the image. Works on a PC, for
traditional BIOS booting or modern UEFI booting. Does not (yet?)
support Secure Boot.

Warning: This is the least robust part of vmdb2.

Step keys:

* `grub` &mdash; REQUIRED; value MUST be one of `uefi` and `bios`, for
  a UEFI or a BIOS boot, respectively. (FIXME: these are valid for a
  PC; not sure what other archs require, if grub even works there.)

* `tag` &mdash; REQUIRED; value is the tag for the root filesystem.

* `efi` &mdash; REQUIRED for UEFI; value is the tag for the EFI
  filesystem.

* `console` &mdash; OPTIONAL; set to `serial` to configure the image
  to use a serial console.

* `device` &mdash; OPTIONAL; which device to install GRUB onto; this
  is needed when installing to a real hard drive, instead of an image.

Example (in the .vmdb file):

    - grub: bios
      tag: root

Same, but for UEFI:

    - grub: uefi
      tag: root
      efi: efi
      console: serial

Install to a real hard disk (named with the `--image` option):

    - grub: uefi
      tag: root
      efi: efi
      image-dev: "{{ image }}"
