Enable again multiboot and multiboot2 modules on EFI builds
    
    Building the multiboot and multiboot2 modules was disabled for EFI builds.
    But that made the menu entries created by the Xen package to stop working
    since they use the multiboot2 module.
    
    The modules were disabled modules because they can be used to bypass the
    Secure Boot mechanism. But it's enough to not include these modules in the
    grub2 EFI binary that's signed, which is the case already in the grub2 pkg.
    
    Having them as modules if the user installs the grub2-efi-x64-modules is
    a valid use case. And since module loading isn't allowed when Secure Boot
    is enabled, it doesn't represent any security threat.
    
    Resolves: rhbz#1703872
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>