src/gpt: use partition count starting with 1 in messages
src/gpt: use partition count starting with 1 in messages

Linux partitions are counted starting with 1. Adjust the messages about
partitions accordingly.

Signed-off-by: Bastian Krause <591dbcb2d48f405f0cb405f5aa6519451f40ad30@pengutronix.de>

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