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.