build: image: fix build breakage of some images

build: image: fix build breakage of some images

Commit 881ed09ee6e2 ("build: create JSON files containing image info")
has removed the crucial empty new line from the image copy step
resulting in the following errors during make function expansion:

 GZ_SUFFIX :=
 bash: GZ_SUFFIX: command not found
 Makefile:86: recipe for target 'openwrt-ath79-generic-tplink_archer-c7-v5-squashfs-sysupgrade.bin' failed

Fixes: 881ed09ee6e2 ("build: create JSON files containing image info")
Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit 0fb23d67f00a18c3f9e712ca97cfc34b38071f4a)