configs/pc_x86_64_efi: use a GPT partition table

Since all EFI-based systems support GPT, this commit changes
pc_x86_64_efi to use a GPT partition table. It shows an example of how
to craft a disk image with GPT partitioning instead of MBR. This is
achieved by means of a post-image script which uses
mkdosfs+mcopy+sfdisk, since genimage is unable to deal with GPT. Long
term, it would be ideal if genimage had GPT support, but until then,
this script shows how to achieve creating a GPT-based disk image.

The script was kept as simple as possible to make it easy to understand
and adapt for other purposes.

The root filesystem location is passed to the kernel by a partition
UUID, so it is possible to boot on QEMU, directly from the disk image,
or dump the image to a physical device.

Signed-off-by: Carlos Santos <casantos@datacom.com.br>
Acked-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>