base-files: sysupgrade: Allow downloading of firmware images using HTTPS

Currently it's only possible to download images over HTTP.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

package/base-files/files/sbin/sysupgrade

@@ -203,7 +203,8 @@ type platform_check_image >/dev/null 2>/dev/null || {
 }
 
 case "$IMAGE" in
-	http://*)
+	http://*|\
+	https://*)
 		wget -O/tmp/sysupgrade.img "$IMAGE"
 		IMAGE=/tmp/sysupgrade.img
 		;;