treewide: when copying a backup file always specify dest name

$CONF_TAR shouldn't be assumed to always point to the sysupgrade.tgz.
This change makes code more generic and allows refactoring $CONF_TAR.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 62dbe361a1)

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

@@ -54,7 +54,7 @@ platform_check_image() {
 platform_copy_config_emmc() {
 	mkdir -p /recovery
 	mount -o rw,noatime /dev/mmcblk0p1 /recovery
-	cp -af "$CONF_TAR" /recovery/
+	cp -af "$CONF_TAR" "/recovery/$BACKUP_FILE"
 	sync
 	umount /recovery
 }