Rename "previous" revocations to "automatic"

When the term previous was introduced for revocations to be
automatically applied there was a hope that everytime a new
revocation was built into shim, the previous revocation could
be applied automatically. Further experience has shown the
real world to be more complex than that. The automatic payload
will realistically contain a set of revocations governed by
both the cadence at which a distro's customer base updates
as well as the severity of the issue being revoked.

This is not a functional change.

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

Author: jsetje