treewide: don't hardcode "sysupgrade.tgz" file name

1) Add BACKUP_FILE and use it when copying an archive to be restored
   after sysupgrade (on the next preinit).
2) Use CONF_TAR for copying backup prepared by the /sbin/sysupgrade

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

target/linux/mediatek/base-files/lib/preinit/79_move_config

@@ -1,6 +1,8 @@
 #!/bin/sh
 # Copyright (C) 2015 OpenWrt.org
 
+. /lib/upgrade/common.sh
+
 RECOVERY_PART=/dev/mmcblk0p1
 
 move_config() {
@@ -11,7 +13,7 @@ move_config() {
 		insmod vfat
 		mkdir -p /recovery
 		mount -o rw,noatime $RECOVERY_PART /recovery
-		[ -f /recovery/sysupgrade.tgz ] && mv -f /recovery/sysupgrade.tgz /
+		[ -f "/recovery/$BACKUP_FILE" ] && mv -f "/recovery/$BACKUP_FILE" /
 		umount /recovery
 	fi
 }