main/grub: fix booting Xen under EFI

Closed William Johansson requested to merge radhus/aports:grub-fix-xen-efi-multiboot2 into master

Need to use multiboot2 while booting Xen under EFI, and not multiboot as is used when generating grub.cfg with grub-mkconfig.

Backport commits included in upstream Grub 2.04.

Diskless setups are not affected, as grub.cfg is generated manually by the mkimage scipts.

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