config: Added schema validation for configuration files

Currently many of the error messages regarding the configuration file
are not really helpful.

To improve reporting this patch adds validation of the configuration
files using the jsonschema module.

Example:

  Forgetting ':' at the end of a layer definition, like this:

    repos:
      meta-iot2000:
        layers:
          meta-iot2000-example
                              ^

  Exception before:

    TypeError: string indices must be integers

  Exception after:

    Validation Error:
     'meta-iot2000-example' is not of type 'object'

     Failed validating 'type' in schema['properties']['repos']['additionalProperties']['properties']['layers']:
         {'additionalProperties': {'oneOf': [{'type': 'null'},
                                             {'type': 'integer'},
                                             {'type': 'boolean'},
                                             {'type': 'string'}]},
          'type': 'object'}

     On instance['repos']['meta-iot2000']['layers']:
         'meta-iot2000-example'

This patch adds 'jsonschema' as an additional dependency.

Signed-off-by: Claudius Heine <ch@denx.de>

kas/__init__.py

@@ -25,6 +25,7 @@
 
 from .__version__ import __version__
 from .__version__ import __file_version__, __compatible_file_version__
+from .configschema import CONFIGSCHEMA
 
 __license__ = 'MIT'
 __copyright__ = 'Copyright (c) Siemens AG, 2017'