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>

docs/userguide.rst

@@ -8,6 +8,7 @@ This projects depends on
 
 - Python 3
 - distro Python 3 package
+- jsonschema Python 3 package
 - PyYAML Python 3 package (optional, for yaml file support)
 
 If you need Python 2 support consider sending patches. The most