bootctl: make --json output normal json

bootctl: make --json output normal json

We would output a sequence of concatenated JSON strings. 'jq' accepts such
output without fuss, and can even automatically build an array with --slurp/-s.
Nevertheless, parsing this format is more effort for the reader, since it's not
"standard JSON". E.g. Python's json module cannot do this out-of-the-box, but
needs some loop with json.JSONDecoder.raw_decode() and then collecting the
objects into an array. Such streaming output make sense in case of logs, where
we stream the output and it has no predefined length. But here we expect at
most a few dozen entries, so it's nicer to write normal JSON that is trivial to
parse.

I'm treating this is a bugfix and not attempting to provide compatibility
backwards. I don't think the previous format was seeing much use, and it's
trivial to adapt to the new one.