firmware-utils: ptgen: add GPT support

firmware-utils: ptgen: add GPT support

Add GPT support to ptgen, so we can generate EFI bootable images.

Introduced two options:
    -g        generate GPT partition table
    -G GUID   use GUID for disk and increase last bit for all partitions

We drop The alternate partition table to reduce size, This may cause
problems when generate vmdk images or vdi images. We have to pad enough
sectors when generate these images.

Signed-off-by: 李国 <uxgood.org@gmail.com>
[fixed compilation on macOS]
Signed-off-by: Petr Štetiar <ynezz@true.cz>