Add kconfiglib-based menu plugin

This uses the kconfiglib python module to implement simple interactive
configuration for complex layers with many kas options and even variable
input. See the menu.py documentation header for details on the Kconfig
format and its translation to kas.

Rather than using the limited menuconfig implementation of kconfiglib,
this comes with an own, python-newt based version. The permits smooth
integration in the workflow, e.g. to offer an option that directly
triggers a build after completing a configuration. And it comes with a
nicer layout.

Unfortunately, python-newt can't be fulfilled via pip. But it is
available as proper package via common distros. So warn if the package
is missing.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

docs/format-changelog.rst

@@ -103,3 +103,9 @@ Changed behavior
 
 - String item ``includes`` are now using repo-relative paths. File-relative is
   still supported by issues a deprecation warning.
+
+Added
+~~~~~
+
+- ``menu_configuration`` key stores the selections done via ``kas menu`` in a
+  configuration file. It is only evaluated by that plugin.

docs/userguide.rst

@@ -10,6 +10,8 @@ This project depends on
 - distro Python 3 package
 - jsonschema Python 3 package
 - PyYAML Python 3 package (optional, for yaml file support)
+- kconfiglib Python 3 package (optional, for menu plugin)
+- NEWT Python 3 distro package (optional, for menu plugin)
 
 To install kas into your python site-package repository, run::
 
@@ -64,6 +66,12 @@ Use Cases
     $ git pull
     $ kas build kas-project.yml
 
+3.  Interactive configuration::
+
+    $ cd $PROJECT_DIR/meta-project
+    $ kas menu
+    $ kas build  # optional, if not triggered via kas menu
+
 
 Plugins
 -------
@@ -86,6 +94,11 @@ typically provides a single command.
 
 .. automodule:: kas.plugins.for_all_repos
 
+``menu`` plugin
+~~~~~~~~~~~~~~~~
+
+.. automodule:: kas.plugins.menu
+
 ``shell`` plugin
 ~~~~~~~~~~~~~~~~
 
@@ -420,3 +433,9 @@ Configuration reference
   * ``<local-conf-id>``: string [optional]
       A string that is added to the ``local.conf``. It operates in the same way
       as the ``bblayers_conf_header`` entry.
+
+* ``menu_configuration``:: dict [optional]
+    This contains user choices for a Kconfig menu of a project. Each variable
+    corresponds to a Kconfig configuration variable and can be of the types
+    string, boolean or integer. The content of this key is typically
+    maintained by the ``kas menu`` plugin in a ``.config.yaml`` file.