main/grub: fix booting Xen under EFI

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.
7 jobs for !2947 with grub-fix-xen-efi-multiboot2 in 9 minutes and 8 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Lint
failed #34349
x86_64 docker-alpine allowed to fail
lint

00:02:53

 
  Build
passed #34354
aarch64 ci-build docker-alpine
build-aarch64

00:03:33

passed #34355
armv7 ci-build docker-alpine
build-armv7

00:03:38

passed #34353
ppc64le ci-build docker-alpine
build-ppc64le

00:06:12

passed #34352
s390x ci-build docker-alpine
build-s390x

00:01:06

passed #34351
x86 ci-build docker-alpine
build-x86

00:03:37

passed #34350
x86_64 ci-build docker-alpine
build-x86_64

00:04:53

 
Name Stage Failure
failed
lint Lint
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