Generate BootLoaderSpec config fragments
    
    The BootLoaderSpec (BLS) defines a scheme where different bootloaders can
    share a format for boot items and a configuration directory that accepts
    these common configurations as drop-in files [0].
    
    Generate BLS snippets at build time that can be copied on kernel install,
    so bootloaders can parse to create menu entries using this information.
    
    [0]: https://www.freedesktop.org/wiki/Specifications/BootLoaderSpec/
    
    Signed-off-by: Peter Jones <pjones@redhat.com>