Add grub2-emu subpackage
    
    GRUB has an user-space program emulator that allows to parse config files
    and execute boot entries using the kexec tool. Add a grub2-emu subpackage
    to install the emulator.
    
    The subpackage is disabled on ppc64le architecture for now since grub2-emu
    fails to build there.
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>