octeon: only copy sysupgrade file if present

this caused

'mv: can't rename '/mnt/sysupgrade.tgz': No such file or directory'

when running sysupgrade -n

Signed-off-by: John Crispin <john@phrozen.org>

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

@@ -5,7 +5,7 @@ move_config() {
 	case "$(cat /tmp/sysinfo/board_name)" in
 		erlite)
 			mount -t vfat /dev/sda1 /mnt
-			mv -f /mnt/sysupgrade.tgz /
+			[ -f /mnt/sysupgrade.tgz ] && mv -f /mnt/sysupgrade.tgz /
 			umount /mnt
 			;;
 	esac