Make 20-grub.install to exit if there is no machine ID set
    
    The kernel-install plugins are called with an environment variable named
    $KERNEL_INSTALL_MACHINE_ID, which is set to the machine ID read from the
    /etc/machine-id file. If the file doesn't exist or is empty, the variable
    is empty and $BOOT_DIR_ABS is set to a temporary directory that's removed
    after all the plugins exit.
    
    So if $KERNEL_INSTALL_MACHINE_ID is not set, just exit the script since
    installing a kernel BootLoaderSpec fragment won't be possible anyways.
    
    Resolves: rhbz#1576573
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>