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>

target/linux/imx6/base-files/lib/upgrade/platform.sh

@@ -17,7 +17,7 @@ enable_image_metadata_check
 
 apalis_copy_config() {
 	apalis_mount_boot
-	cp -af "$CONF_TAR" /boot/
+	cp -af "$CONF_TAR" "/boot/$BACKUP_FILE"
 	sync
 	umount /boot
 }