menu plugin: track root repo dir

This patch adds a new top-level schema node `_source_dir` which is used
to track the top repo dir on auto-generated kas files. This node is
generated by the kas menu plugin and provides the absolute path to the
top repo dir at time of invoking the plugin. When later calling any other
kas operation that performs a checkout, this node is evaluated and the
top repo dir is set accordingly. This tracking is required when the
build command is executed from another dir than the top repo dir, as the
repo-dir cannot be computed by kas in this case (the .config.yaml file
is in the KAS_WORK_DIR, not in the repo dir).

Proposed-by: Peter Hoyes <Peter.Hoyes@arm.com>
Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
[Jan: aligned format-changelog text, simplified load_config returning]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

docs/format-changelog.rst

@@ -138,3 +138,5 @@ Added
 ~~~~~
 
 - The ``overrides`` top-level entry can be used to pin floating repo refspecs.
+- ``_source_dir`` top-level entry is auto-generated when using the menu plugin
+  and provides the path to the top repo at time of invoking the plugin.

docs/userguide.rst

@@ -533,3 +533,8 @@ Configuration reference
     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.
+
+* ``_source_dir``:: string [optional]
+    This entry is auto-generated by the menu plugin and provides the path to
+    the top repo at time of invoking the plugin. It must not be set
+    manually and might only be defined in the top-level ``.config.yaml`` file.