repart: Add --split option to generate split artifacts

repart: Add --split option to generate split artifacts

For use with sysupdate or other systemd tooling, it's useful to be
able to generate split artifacts from disk images, where each
partition is written to a separate file. Let's support this with
a --split switch for repart and a SplitName= configuration option.

--split enables split artifacts generation, and SplitName= configures
for which partition to generate split artifacts, and which suffix to
add to the split artifact name.

For SplitName=, we add support for some extra specifiers, more specifically
the partition Type UUID and the partition UUID.