Allow only creating unified ISO for specified arch

Fixes: https://pagure.io/pungi/issue/1393
JIRA: RHELCMP-807
Signed-off-by: Haibo Lin <hlin@redhat.com>

pungi/scripts/create_unified_isos.py

@@ -18,11 +18,18 @@ def parse_args():
     parser.add_argument(
         "compose", metavar="<compose-path>", nargs=1, help="path to compose",
     )
+    parser.add_argument(
+        "--arch",
+        metavar="<arch>",
+        dest="arches",
+        action="append",
+        help="only generate ISOs for specified arch",
+    )
 
     return parser.parse_args()
 
 
 def main():
     args = parse_args()
-    iso = UnifiedISO(args.compose[0])
+    iso = UnifiedISO(args.compose[0], arches=args.arches)
     iso.create(delete_temp=True)