Rework mapviewer and mapviewer-demo

Rework mapviewer and mapviewer-demo

Rework the mapviewer and mapviewer-demo recipes to get mapviewer
working out of the box for the cluster demo:
- Remove the old on/off scripts, as the intent is that the cluster
  demo work out of the box on an image built with the
  agl-cluster-demo-support feature.
- The separate weston systemd unit has been replaced with a drop-in
  over-ride file.
- A new systemd unit is installed to configure the network connection
  to the cluster board. This is required now because the weston unit
  no longer runs as root.
- A systemd drop-in is added to have afm-api-windowmanager@.service
  depend on weston-ready. This is a bit of a workaround ATM, as the
  windowmanager service was consistently failing due to Weston
  taking longer to start with the gst-record feature enabled. It is
  likely that making this more generic should be investigated.
- The mapviewer systemd unit has been updated.  Its Install target
  is now afm-user-session@.target; in my testing this seemed the
  least invasive solution, as there are some dependency loop issues
  that currently prevent adding it to multi-user.target. As well,
  its dependencies have been updated from weston and HomeScreen to
  afm-api-windowmanager@, which is not entirely ideal, but is about
  the best that can be done ATM given that mapviewer interacts with
  the other windowmanager users via Weston, but is not an app
  framework application.

Change-Id: I8826e670ae156edd461cc657acefc86e7836a916
Signed-off-by: Scott Murray <scott.murray@konsulko.com>