[PULL][Trusty] arm64: Add EFI stub support

[Dann Frazier]

Ubuntu currently provides arm64-efi cloud images, but the 14.04 LTS
images aren't actually bootable due to the lack of EFI stub support.
This means there is currently no Ubuntu LTS option for users deploying
guests on ARM64-based clouds, and that's just not cool, man.

  git://kernel.ubuntu.com/dannf/trusty-xgene.git@arm64-efi

Prebuilt packages are available in ppa:dannf/arm64efi.

This is based on an earlier backport from Ming Lei, but modified to
minimize the changes (read: regression risk) to x86. There are no
changes under arch/x86, and all of the arch-indep changes are adding
new interfaces that x86 doesn't use.

With these changes, the trusty cloud images are now bootable. However,
they do not have automatic console detection (you have to manually add
console=ttyAMA0), and do not yet have an EFI RTC driver - that work is
up next.

I've regression tested on X-Gene/u-boot and on the QEMU arm64 virt
model booted w/ -kernel to make sure it doesn't break non-EFI systems.
Thanks to Rod Smith for regression testing on an EFI-mode mac and
within virtualbox to confirm I didn't break EFI mode on x86.