src/install: provide some hints in log about using variant image
src/install: provide some hints in log about using variant image

This might ease debugging image variant installations

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

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