This reverts commit dcff6802.

A bug in batman-adv can lead to a large amount of management traffic being
exchanged between nodes when the multicast optimizations are enabled,
effectively making the mesh unusable. It's safer to disable the feature
for now, until we have a real fix.

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.