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/configschema.py

@@ -78,6 +78,12 @@ CONFIGSCHEMA = {
         'distro': {
             'type': 'string',
         },
+        'env': {
+            'type': 'object',
+            'additionalProperties': {
+                'type': 'string',
+            },
+        },
         'target': {
             'oneOf': [
                 {