mender-convert: Add support for GPT partition tables

mender-convert (v2) does not currently produce GPT images when the which would be required according to the UEFI specification.

This primarily applies to x86_64 as GPT images have been shown not work well together with ARM devices which typically offset the bootloader on the disk which overlaps with the GPT information.

You can read more about this in this community thread.

Acceptance criteria:

• On x86 platforms the default is to create an GPT image
• On ARM platforms the default is to create an MBR image
• Options should be provided to override either