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` — 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` — REQUIRED; value is the tag for the root filesystem.

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

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

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

* `quiet` — OPTIONAL; should the kernel be configured to boot
  quietly? Default is no.

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 }}"