Add Secure Boot Advanced Targeting (SBAT) specification document

Add Secure Boot Advanced Targeting (SBAT) specification document

SBAT is a new Generation Number Based Revocation meant to replace the DBX
Revocation List Files mechanism. It is more flexible and allow to revoke
sets of binaries, instead of having to list all of them as with the DBX.

Metadata that includes the vendor, product family, product, component,
version and generation are added to artifacts in a .sbat section. This
is protected by the digital signature and so it cannot be tampered.

Signed-off-by: Jan Setje-Eilers <jan.setjeeilers@oracle.com>
Signed-off-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Gary Lin <glin@suse.com>