Add gateway demo

Add gateway demo

Changes:
- Add recipe for AGL VSS to MQTT proxy daemon
- Update agl-vcar.dbc CAN database definition to add signals useful
  for demoing the proxy (per V2C EG discussion).
- Add a patch to kuksa-dbc-feeder to allow sensor signal updates to
  generate CAN messages in output mode.
- Add VSS vspec variants to define the desired CAN input and output
  support for the various kuksa-dbc-feeder instances in the default
  and full gateway demos.
- Add configurations for kuksa-dbc-feeder for the running the default
  gateway demo with CAN output from a demo control panel instance on
  a single CAN interface, as well as a fuller setup with a second
  kuksa-dbc-feeder running against a second CAN interface on the
  gateway to handle the demo steering wheel and HVAC support.
- Add gateway demo specific configuration files for various KUKSA.val
  databroker clients to override the databroker location.
- Add agl-gateway-demo and agl-gateway-demo-preconfigured images for
  the default and full demos.
- Add *-preconfigured-gateway image flavors for the Flutter IVI, IC,
  and KVM demo images that support running with the databroker on
  the gateway.

NOTES:
- The *-preconfigured-gateway images assume the gateway has an IP
  address of 192.168.10.4.
- Required changes to the agl-demo-control-panel application and the
  addition of a agl-ivi-demo-control-panel-preconfigured-gateway
  image will come in a subsequent change.

Bug-AGL: SPEC-5107, SPEC-5138

Change-Id: I9797aa72737af7af3d791a5151198f80b6d90e0d
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/29906
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>