amend! secboot: use uuid of luks2 instead of partition

secboot: use uuid of luks2 instead of partition

`by-partuuid` does not make much sense because it uselessly assumes
that it is a partition. Conceptually we should not care about it.  It
also makes the resolution more complex as we need to fetch information
about the device which we do not really need at this point.

It is more common to resolve by filesystem UUID than part UUID. For
instance cryptsetup accepts path as
`UUID=deadbeef-dead-dead-dead-deaddeafbeef`. But it does not accept
this kind of syntax for partitions.