ar71xx: sysupgrade: accept ath79 combined-image

There is md5 sum of whole image embedded in combined-image header which
is checked on sysupgrade. The check will fail for ath79 images which
may have embedded metadata. This is because metadata are appended after
the combined image is created. To allow smooth transition from ar71xx to
ath79, strip metadata before calculating md5 sum for whole image.

Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>
(cherry picked from commit 8b4109c2b4d60495d046157d1baca9b1cdbf8dc8)

target/linux/ar71xx/base-files/lib/upgrade/platform.sh

@@ -394,7 +394,7 @@ platform_check_image() {
 		}
 
 		local md5_img=$(dd if="$1" bs=2 skip=9 count=16 2>/dev/null)
-		local md5_chk=$(dd if="$1" bs=$CI_BLKSZ skip=1 2>/dev/null | md5sum -); md5_chk="${md5_chk%% *}"
+		local md5_chk=$(fwtool -q -t -i /dev/null "$1"; dd if="$1" bs=$CI_BLKSZ skip=1 2>/dev/null | md5sum -); md5_chk="${md5_chk%% *}"
 
 		if [ -n "$md5_img" -a -n "$md5_chk" ] && [ "$md5_img" = "$md5_chk" ]; then
 			return 0