util/grub-mkrescue: Preserve a copy of the EFI bootloaders on the ISO 9660 file system

util/grub-mkrescue: Preserve a copy of the EFI bootloaders on the ISO 9660 file system

To enable file system transposition support for UEFI, we also must ensure that
there exists a copy of the EFI bootloaders, that are currently embedded in the
efi.img for xorriso, at their expected UEFI location on the ISO 9660 file system.

This is accomplished by removing the use of a temporary directory to create the
efi/ content, to instead place it at the root of the ISO 9660 content.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>