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/__version__.py

@@ -28,5 +28,5 @@ __copyright__ = 'Copyright (c) Siemens AG, 2017'
 __version__ = '0.14.0'
 
 # Please update docs/format-changelog.rst when changing the file version.
-__file_version__ = 5
+__file_version__ = 6
 __compatible_file_version__ = 1

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

kas/configschema.py

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

kas/libkas.py

@@ -306,7 +306,13 @@ def get_build_environ(config, build_dir):
         except ValueError:
             pass
 
+    conf_env = config.get_environment()
+
     env_vars = ['SSTATE_DIR', 'DL_DIR', 'TMPDIR']
+    env_vars.extend(conf_env)
+
+    env.update(conf_env)
+
     if 'BB_ENV_EXTRAWHITE' in env:
         extra_white = env['BB_ENV_EXTRAWHITE'] + ' '.join(env_vars)
         env.update({'BB_ENV_EXTRAWHITE': extra_white})