Bug#915091: arm-efi/arm64-efi initrd handling fixes

[Leif Lindholm]

Package: grub2
Severity: important

Two major aspects of grub initrd handling on arm* have been addressed
upstream, but are not part of any release:
- Adhering to the initrd/initramfs placement rules
- Ensuring fdt address/size cells are set to 64-bit

The former can prevent booting a system with lots of RAM or holes
between RAM banks.

The latter corresponds to linux kernel commit ae8a442dfdc4 and brings
the behaviour of the arm* EFI Linux loader in line with the behaviour
of booting the EFI Stub kernel directly.

Addressing the placement rules on arm includes moving to share the
linux loader with the arm64 port, which has minor fallout across other
architectures.

Already upstream patches mildly reworked to apply on current state of
Debian grub can be found at:
https://git.linaro.org/people/leif.lindholm/grub.git/log/?h=debian-buster-arm-fixes

(This branch also deals with problems reported in
https://bugs.debian.org/909420 and https://bugs.debian.org/907596.)