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/imx6/base-files/lib/preinit/79_move_config

@@ -2,6 +2,7 @@
 
 . /lib/imx6.sh
 . /lib/functions.sh
+. /lib/upgrade/common.sh
 
 move_config() {
 	local board=$(board_name)
@@ -10,7 +11,7 @@ move_config() {
 	apalis*)
 		if [ -b $(bootpart_from_uuid) ]; then
 			apalis_mount_boot
-			[ -f /boot/sysupgrade.tgz ] && mv -f /boot/sysupgrade.tgz /
+			[ -f "/boot/$BACKUP_FILE" ] && mv -f "/boot/$BACKUP_FILE" /
 			umount /boot
 		fi
 		;;