Platform: corstone1000: Generate a gpt flash image

Create the corstone1000 flash image with the gpt layout via a script
The script uses dd command to create the disk image
and uses the sgdisk tool to write the gpt layout
dd is used again to write binaries to the disk image

Signed-off-by: Mohamed Omar Asaker <mohamed.omarasaker@arm.com>
Change-Id: I38c34f27c1bad1d450ca54b4fffd24a66ccd4746