meta/files: Add a jsonschema for bitbake-setup configuration files

meta/files: Add a jsonschema for bitbake-setup configuration files

This schema is a bit loose and should validate any configuration files working with
bitbake-setup but, also, some broken ones:
* If present, a "bb-layer" can be an empty array.
* bb-setup need at least one of "bb-layers" or "oe-template", that is
  not enforced in the current schema.
* bb-setup accepts "configurations = []" but it results in a impossible
  choice in interactive mode. This is rejected by the schema.
* In each configuration, "name" and "description" are optional but the
  flatten configuration must have them. This is not enforced by the
  schema.

To test a configuration files against this schema: (for exemple to
validate bitbake default registry)
   $ pip install check-jsonschema
   $ check-jsonschema -v --schemafile meta/files/bitbake-setup.schema.json bitbake/default-registry/configurations/*
  ok -- validation done
  The following files were checked:
    bitbake/default-registry/configurations/oe-nodistro.conf.json
    bitbake/default-registry/configurations/poky-master.conf.json

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>