grub2 should install in mixed-mode for multi-arch x86_64/i386 support to allow EFI-32 boots

[Jack Howarth]

Description of problem: Currently the existing Ubuntu Live installers can't boot from a USB key on legacy Apple hardware like a MacBook Pro 2,1 which lacks EFI-64 firmware with only EFI-32 firmware. The next Ubuntu release should adopt the same mixed-mode multi-arch installer approach as taken by Debian in their Jessie
https://cdimage.debian.org/debian-cd/current/multi-arch/iso-cd/debian-8.6.0-amd64-i386-netinst.iso installer.

Version-Release number of selected component (if applicable):

How reproducible:
Always

Steps to Reproduce:
1. Attempt to boot a USB drive or memory key with the Ubuntu 16.10 x86_64 or i386 Live installer on any of the following Apple hardware which lacks EFI-64 firmware and only has EFI-32 firmware...

MacBook2,x
MacBook3,x
MacBook4,x
MacBookPro2,x
MacBookAir1,1
MacMini1,x with C2D
MacMini2,x
iMac4,x with C2D
iMac5,x

MacPro1,x
MacPro2,x

2.You will find that the option key EFI boot selector shows no icon for the attached USB drive or key.

3.Prepare a USB drive or key with the debian-8.6.0-amd64-i386-netinst.iso installed on it.

Actual results:
The boot selector will show the boot EFI for the Debian Jessie multi-arch installer on a USB device mounted on an EFI-32 firmware Mac, but not for any of the available Fedora releases.

Expected results:
I expected the Ubuntu releases to be able to boot the x86_64 linux from a USB device.

Additional info:
The Debian Mixed-Mode support is described on https://wiki.debian.org/UEFI as follows...

Support for mixed-mode systems: 64-bit system with 32-bit UEFI
Some systems have been released containing 64-bit Intel Atom CPUs (such as the Bay Trail), but unfortunately use 32-bit UEFI firmware with no BIOS compatibility mode. Using the 32-bit UEFI x86 support, an i386 installation should be possible on these machines but it won't make the most of the 64-bit hardware.

Debian Jessie (8.0) was the first Linux distribution to include full support for mixed-mode UEFI installation on these machines. The multi-arch installation media (available in netinst and DVD form) include the UEFI boot loaders necessary for both i386 and amd64 boot. By selecting "64-bit install" from the initial boot menu, debian-installer will install a 64-bit (amd64) version of Debian. The system will automatically detect that the underlying UEFI firmware is 32-bit and will install the appropriate version of grub-efi to work with it.

Note that a 64-bit Graphical install of x86_64 Debian Jessie Linux using the multi-arch iso installs...

/boot/efi/EFI/debian/boot.efi
/boot/efi/EFI/debian/bootia32.efi

to allow the x86_64 Linux kernel to boot on the EFI-32 motherboard.