Add support for forcing EFI installation to the removable media path

Add support for forcing EFI installation to the removable media path

Add an extra option to grub-install "--force-extra-removable". On EFI
platforms, this will cause an extra copy of the grub-efi image to be
written to the appropriate removable media patch
/boot/efi/EFI/BOOT/BOOT$ARCH.EFI as well. This will help with broken
UEFI implementations where the firmware does not work when configured
with new boot paths.

Also added new debconf logic to add this extra option to grub-install
calls when grub2/force_efi_extra_removable is set true. This allows
other programs like d-i / grub-installer to configure this for general
use.

Provides part of the fix for #767037

[ ijc -- included debian/po/* update ]