src/main: move bundle path resolving to own function
src/main: move bundle path resolving to own function

This will allow using the same code at all locations handling bundle
path arguments.

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

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