sysmocom-backup-data: Avoid an absolute symlink to fix restoring

When restoring the backup the rootfs is mounted in /rootfs/data
and the symlink to the real file would then not work. Attempt to
change the directory first to get a relative symlink.

Untested change and needed for rauc

recipes-apps/sysmocom-backup-data/files/sysmocom-backup-data

@@ -70,7 +70,8 @@ do_backup_files() {
 	tar -rf $BACKUP_FILE --transform='s,^,info/,' -C $NAME installed_packages mac_addr
 
 	# 6. Create stable link
-	ln -sf $BACKUP_FILE /data/sysmocom-backup.tar
+	cd /data/
+	ln -sf `basename $BACKUP_FILE` sysmocom-backup.tar
 
 	# 76
 	echo "The backup was stored to $BACKUP_FILE"