wic: fix processing of --use-uuid

Processing of this option was lost during recent change of
wks parsting. It was discovered during the work on booting
wic images under qemu. Now, when -use-uuid is fixed it's
possible to specify root partition by partition uuid.
This will be done in the following commit.

(From OE-Core rev: b4882e0b84d7fd4c85ee95386e94722485eafc2b)

(From OE-Core rev: 73e9e3f150bf2de9b27c2ccc73e3dee334ee73fe)

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

scripts/lib/wic/partition.py

@@ -57,7 +57,10 @@ class Partition(object):
         self.size = args.size
         self.source = args.source
         self.sourceparams = args.sourceparams
+        self.use_uuid = args.use_uuid
         self.uuid = args.uuid
+        if args.use_uuid and not self.uuid:
+            self.uuid = str(uuid.uuid4())
 
         self.lineno = lineno
         self.source_file = ""