mkefidisk.wks: use partition UUID and GPT partition table

mkefidisk.wks: use partition UUID and GPT partition table

This is a preparation to use mkefidisk as a default wks for
genericx86* BSPs. This change enables usage of partition UUID
instead of device name to specify root partition in kernel
command line. It should make images to boot on devices with
boot device names that differ from what's mentioned in wks file.

(From OE-Core rev: 23cca700870230b46d251086441136e99659ef12)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>