generic: busybox: enable sha512sum to workaround segfault issue

sha512sum doesn't add much code that is not also used by sha256sum, but the
change of the configuration hides the segfault issue described in:

  https://bugs.lede-project.org/index.php?do=details&task_id=822

While the issue only seemed to affect dhcpv6.script, it would clutter /tmp
with coredumps, eventually leading to OOM.