classes: bundle: Add option to build casync bundle
classes: bundle: Add option to build casync bundle

Add an option to build a casync bundle alongside the regular bundle.
This requires setting an appropriate keyring. Activate the generation of
casync bundles by setting RAUC_CASYNC_BUNDLE to "1".

Signed-off-by: Martin Schwan <0b4b17d835f86924185dc5770bd021250822e547@phytec.de>

rauc/meta-rauc,rauc/meta-rauc,rauc/meta-rauc,rauc/meta-rauc