Rework demo packagegroup and images

Rework the agl-ivi-demo-platform* images to reduce duplication,
and move the additions for the agl-demo-cluster-support and
agl-demo-preload features.

Changes:
- Pull in agl-image-ivi.bb in the crosssdk image rather than
  duplicating its contents.  This means the package additions
  from the demo features will be reflected in the SDK, which
  seems more in line with general expectations of how Yocto SDKs
  work.
- New packagegroups are created for the agl-demo-cluster-support
  and agl-demo-preload features, and they are pulled into the
  agl-ivi-demo-platform / agl-demo-platform image using features
  in IMAGE_FEATURES.  This avoids changing the contents of various
  packagegroups based on AGL_FEATURES and seems more futureproof
  with respect to binary packaging.
- ALLOW_EMPTY has been removed in a few packagegroup recipes, as
  it does not seem to make sense if the packagegroup will never
  be empty.
- The cluster-dashboard-demo-config recipe pulled into the
  agl-demo-cluster-support packagegroup has been added back, it
  was accidentally removed in the application framework cleanup.

Bug-AGL: SPEC-4424

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I114b77ded6c47414270acfc3cd06e998dbcc4cef

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