udev: add udev properties that point to verity/verity sig metadata partitions from...

udev: add udev properties that point to verity/verity sig metadata partitions from data partitions

This extends the dissect_image builtin to actually add device node
references to the device nodes where the associated data is placed, if
we can find it.

This is kept very generic, and independent from the roothash properties
and suchlike, since it makes sense to make it possible to set these
properties also independently of the dissect-image builtin.

The device path is a /dev/disk/by-diskseq/ symlink, so that we have
stable reference that are not subject to dev_t reuses.