gluon-announce, ...: don't output empty lists where not appropriate

Always output empty objects or nothing at all where objects are expected, but
no elements exist.

Also remove a few unneeded "requires", a few basic modules are provided by
announce.lua by default.

This updates batman-adv to 2015.0.

This also fixes the TL-WA830RE v1 model name.

Export Mode -> Expert Mode

final image name is shown as tp-link-tl-wa830re-v10

primary mac is the one printed on the case

Fixes #373

15 is the default in batman-adv-legacy, the new default 30 in newer batman-adv
versions is too high in larger networks.

docs/site-example: explain how to re-enter configmode

Fixes #347

docs/autoupdater: depends on ipv6 reachable update-servers

fixes #365