Update docs to match bitbake variable changes

BB_ENV_EXTRAWHITE is BB_ENV_PASSTHROUGH_ADDITIONS in newer Bitbake
versions. Keep reference to old variable name for users on a old Bitbake
version.

Signed-off-by: Jasper Orschulko <[email protected]>
Signed-off-by: Jan Kiszka <[email protected]>

docs/userguide/project-configuration.inc

@@ -290,14 +290,15 @@ Configuration reference
 
 ``env``: dict [optional]
   Contains environment variable names with either default values or ``null``.
-  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.
+  These variables are made available to bitbake via
+  ``BB_ENV_PASSTHROUGH_ADDITIONS`` (``BB_ENV_EXTRAWHITE`` in older Bitbake
+  versions) and can be overwritten by the variables of the environment in
+  which kas is started.
   Either a string or nothing (``null``) can be assigned as value.
   The former one serves as a default value whereas the latter one will lead
-  to add the variable only to ``BB_ENV_EXTRAWHITE`` and not to the
-  environment where kas is started. Please note, that ``null`` needs to be
-  assigned as the nulltype (e.g. ``MYVAR: null``), not as 'null'.
+  to add the variable only to ``BB_ENV_PASSTHROUGH_ADDITIONS`` and not to
+  the environment where kas is started. Please note, that ``null`` needs to
+  be assigned as the nulltype (e.g. ``MYVAR: null``), not as 'null'.
 
 ``task``: string [optional]
   Contains the task to build by bitbake. Can be overwritten by the