Initial population

Content imported from the old test distro, with the
following modifications:

* Removed most custom configuration files
* All open-source layers fully included under layers/
* Secureboot support removed
* Layout reworked to eliminate extra subdirectory for
  core layer
* Added meta-tegra-support layer for holding common
  metadata for use by all distro layers
* Reworked setup-env script to be usable for multiple
  distros sharing the repository
* Added demo image recipes and packagegroups

Signed-off-by: Matt Madison <matt@madison.systems>