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/libcmds.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
@@ -225,7 +225,7 @@ class SetupEnviron(Command):
         return 'setup_environ'
 
     def execute(self, ctx):
-        ctx.environ.update(get_build_environ())
+        ctx.environ.update(get_build_environ(ctx.config.get_build_system()))
 
 
 class WriteBBConfig(Command):