grub2: add usage notes for grub2 arm and aarch64

grub2: add usage notes for grub2 arm and aarch64

Add notes to test grub2 running on ARM using qemu. The arm section
describes how to run it using u-boot and aarch64 shows how to do it
using efi, which is similar to what has to be done for x86_64.

The source for OVMF builds is also changed to
https://www.kraxel.org/repos/jenkins/edk2/ which is the source for
nightly builds (as rpms but which can be extracted in any distribution),
as the sourceforge link provided only very old builds.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
[Thomas:
 - formatting fixes
 - simplify the AArch64/EFI example by using the aarch64_efi_defconfig]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>