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>
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