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.

This reverts commit fcce9e52.

The x86-generic kernel does not fit into 4MB.

This will allow upgrading using the broken sysupgrade scripts in OpenWrt
CC without losing the configuration.

Fixes #1010

Switch sha512sum to sha256sum to match updated autoupdater.