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>
(cherry picked from commit bf39047872)

target/linux/brcm2708/base-files/lib/preinit/79_move_config

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