Fix deprecated option hdimage.gpt in image configuration file

[agambier]

During final image creation, the genimage tool outputs the following info message.

INFO: hdimage(sdcard.img): The option 'gpt' is deprecated. Use 'partition-table-type' instead

More information on the project's github : https://github.com/pengutronix/genimage#hdimage

[tpetazzoni]

Thanks, merged with a few tweaks to the commit log!