blsuki: Add blscfg command to parse Boot Loader Specification snippets

blsuki: Add blscfg command to parse Boot Loader Specification snippets

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.

The BLS Specification: https://uapi-group.org/specifications/specs/boot_loader_specification/

Signed-off-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Will Thompson <wjt@endlessm.com>
Signed-off-by: Alec Brown <alec.r.brown@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>