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>

docs/format-changelog.rst

@@ -48,3 +48,12 @@ Changed behavior
 
 - Using ``multiconfig:*`` targets adds appropriate ``BBMULTICONFIG`` entries to
   the ``local.conf`` automatically.
+
+Version 6
+---------
+
+Added
+~~~~~
+
+- ``env`` key now allows to pass custom environment variables to the bitbake
+  build process.

docs/userguide.rst

@@ -294,6 +294,11 @@ Configuration reference
     ``core-image-minimal``. Space is used as a delimiter if multiple targets
     should be specified via the environment variable.
 
+* ``env``: dict [optional]
+    Contains environment variable names with the default values. These
+    variables are made available to bitbake via ``BB_ENV_EXTRAWHITE`` and can
+    be overwritten by the variables of the environment in which kas is started.
+
 * ``task``: string [optional]
     Contains the task to build by bitbake. Can be overwritten by the
     ``KAS_TASK`` environment variable and defaults to ``build``.