src/bundle: split up bundle creation and bundle signing
src/bundle: split up bundle creation and bundle signing

This will allow implementing different bundle creation methods as well
as using the sign function for resigning, etc.

Signed-off-by: Enrico Jorns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

rauc/rauc,rauc/rauc,ejoerns/rauc,jluebbe/rauc,ejoerns/rauc,jluebbe/rauc,jluebbe/rauc,ejoerns/rauc,rauc/rauc