board/*/genimage.cfg: use partition-table-type = "gpt" for genimage-15
Genimage 15 deprecated the gpt option and now prints a warning when it is
used:
INFO: hdimage(sdcard.img): The option 'gpt' is deprecated. Use 'partition-table-type' instead
So change the genimage configuration files to use that instead.
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>