Implement file version and a check when using it

If the configuration file structure changes over time, it might
introduce incompatibilities.

This patch introduces versioning of configuration file.

Configuration files needs to add a 'version' entry into their 'header'
like this:

header:
  version: '0.9'

After loading this file, the version is checked agains kas version.

Every version 'M.m.p' of kas is allowed to load configuration files with
version 'M.m' and all versions backwards up to and including
the '__compatible_version__' from kas/__version__.py

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

kas/__init__.py

@@ -24,6 +24,7 @@
 """
 
 from .__version__ import __version__
+from .__version__ import __compatible_version__
 
 __license__ = 'MIT'
 __copyright__ = 'Copyright (c) Siemens AG, 2017'