Add build_system property to config file

This allows to pre-select the build system, specifically avoiding
confusion when kas-container is accidentally not called with --isar for
an isar config. For that, build_system needs to be defined in the
lop-level config file passed to kas-container.

Theoretically, this also allows to combine layers which have both
oe-init-build-env and isar-init-build-env scripts.

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

docs/format-changelog.rst

@@ -86,3 +86,11 @@ Added
   property ``refspec`` and the repository patch property ``repo``. These
   default values will be used if the appropriate properties are not defined
   for a given repository or patch.
+
+Version 10
+----------
+
+Added
+~~~~~
+
+- ``build_system`` property to pre-select OE or Isar.

docs/userguide.rst

@@ -276,6 +276,15 @@ Configuration reference
       * ``file``: string [required]
           The path to the file relative to the root of the repository.
 
+* ``build_system``: string [optional]
+    Defines the bitbake-based build system. Known build systems are
+    ``openembedded`` (or ``oe``) and ``isar``. If set, this restricts the
+    search of kas for the init script in the configured repositories to
+    ``oe-init-build-env`` or ``isar-init-build-env``, respectively. If
+    ``kas-container`` finds this property in the top-level kas configuration
+    file (includes are not evaluated), it will automatically select the
+    required container image and invocation mode.
+
 * ``defaults``: dict [optional]
     This key can be used to set default values for various properties.
     This may help you to avoid repeating the same property assignment in