Use -O instead of -o for ubuntu-image classic builds.

live-build/auto/build

@@ -35,9 +35,13 @@ if [ "${IMAGEFORMAT:-}" = "ubuntu-image" ]; then
 		make -C "config/$PREFIX-gadget"
 		ubuntu-image classic $UBUNTU_IMAGE_ARGS \
 			-p $PROJECT -a $ARCH --subarch $SUBARCH \
-			-o "$PREFIX".img config/$PREFIX-gadget/install
+			-O output config/$PREFIX-gadget/install
+		# XXX: currently we only have one image generated, but really
+		#  we should be supporting more than one for models that
+		#  define those.
+		mv output/*.img "$PREFIX".img
 		xz -0 -T4 "$PREFIX".img
-		mv filesystem.manifest "$PREFIX".manifest
+		mv output/filesystem.manifest "$PREFIX".manifest
 	fi
 
 	exit 0