implementation of environment variable passthrough

With this its possible to create custom environment variables within the kas
configuration file, that are passed through to the build environment:

    env:
      VARIABLE_NAME: "default variable value"

Those variables can be overwritten by variables of the environment kas is
started in:

    $ VARIABLE_NAME="overwritten variable value" kas shell ...

Those variables are also added to the 'BB_ENV_EXTRAWHITE' variable list to make
them available to bitbake.

This is useful for instance storing sensitive information, that should not be
part of the configuration files or bitbake recipes.

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

kas/config.py

@@ -277,6 +277,14 @@ class Config:
         return os.environ.get('KAS_DISTRO',
                               self._config.get('distro', 'poky'))
 
+    def get_environment(self):
+        """
+            Returns the configured environment variables from the configuration
+            file, with possible overwritten values from the environment.
+        """
+        env = self._config.get('env', {})
+        return {var: os.environ.get(var, env[var]) for var in env}
+
     def get_multiconfig(self):
         """
             Returns the multiconfig array as bitbake string