Add example IC EG container demo image

Changes:
- Add a meta-agl-ic-container dynamic layer hierarchy to hold
  metadata dependent on the agl-ic-container feature.
- Add configuration to allow building the agl-demo-platform image
  as a guest container with agl-ic-container.
- Add a agl-lxc-demo-platform image that uses agl-demo-platform as
  the IVI guest container.

NOTE: While this image boots, further integration of IC EG work is
required to actually successfully boot the agl-demo-platform guest.

Bug-AGL: SPEC-4703

Change-Id: I60a46b6ae3c950c4aa25554639d7828462745bfa
Signed-off-by: Scott Murray <scott.murray@konsulko.com>