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>

kas/config.py

@@ -1,6 +1,6 @@
 # kas - setup tool for bitbake based projects
 #
-# Copyright (c) Siemens AG, 2017-2018
+# Copyright (c) Siemens AG, 2017-2020
 #
 # Permission is hereby granted, free of charge, to any person obtaining a copy
 # of this software and associated documentation files (the "Software"), to deal
@@ -53,6 +53,12 @@ class Config:
         self.handler = IncludeHandler(self.filenames)
         self.repo_dict = self._get_repo_dict()
 
+    def get_build_system(self):
+        """
+            Returns the pre-selected build system
+        """
+        return self._config.get('build_system', '')
+
     def find_missing_repos(self):
         """
             Returns repos that are in config but not on disk