Document how revocations can be delivered

Revocation metadata has been consolidated into SbatLevel_Variable.txt and
can be delivered both built into shim as well as via revocations_sbat.efi
binaries. This adds a short text file describing how revocation levels
can be built into these components and delivered.

Signed-off-by: Jan Setje-Eilers <[email protected]>

Author: jsetje

Delivering_Sbat_Revocations.md

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+When new sbat based revocations become public they are added to
+https://github.com/rhboot/shim/blob/main/SbatLevel_Variable.txt They
+are identified by their year, month, day, counter YYYYMMDDCC field in
+the header.
+
+If secure boot is disabled, shim will always clear the applied
+revocations.
+
+shim binaries will include the opt-in latest revocation payload
+available at the time that they are built. This can be applied by
+running mokutil --set-sbat-policy latest and rebooting with the new
+shim binary in place. A shim build can also specify a
+-DSBAT_AUTOMATIC_DATE=YYYYMMDDCC on the command line which will
+include and automatically apply that revocation. shim will never
+downgrade a revocation. The only way to roll back is to disable secure
+boot, load shim to clear the revocations and then re-apply the desired
+level.
+
+In addition to building revocation levels into shim, they can also be
+delivered via a revocations_sbat.efi binary. These binaries can be
+created from the https://github.com/rhboot/certwrapper
+repository. This repository uses the same
+https://github.com/rhboot/shim/blob/main/SbatLevel_Variable.txt file
+as the source of the revocation metadata. Both
+SBAT_LATEST_DATE=YYYYMMDDCC and SBAT_AUTOMATIC_DATE=YYYYMMDDCC can be
+specified there. These files need to be signed with a certificate that
+your shim trusts. These files can be created without the need to
+deliver a new shim and can be set to have shim automatically apply a
+new revocations whey they are delivered into the system partition.