image: Add a post-MBR gap to the default image.

image: Add a post-MBR gap to the default image.

The generic 'efi-disk-image' needs to be bootable on systems without EFI. To
do that, GRUB is installed in the post-MBR gap. Make sure that the first
partition starts with an offset, to make this gap large enough for GRUB.

* gnu/system/image.scm (root-offset, root-label): New variables,
(esp-partition): use 'root-offset' as the partition offset,
(root-partition): use 'root-label' as the partition label.