repart: add parameter to attach offline verity signatures

repart: add parameter to attach offline verity signatures

Add --join-signature=hash:sig - when a verity signature partition
has been deferred in a previous run, this allows attaching a signature
that was created offline, for example on a build system like OBS where
the private key is not available to the build process.

Can be specified multiple times, the right partition to act upon will
be selected by matching the data+verity partitions UUIDs with the
provided roothash(es)