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.

targets/generic

@@ -4,6 +4,7 @@ config 'CONFIG_ALL_NONSHARED=y'
 
 config 'CONFIG_BUSYBOX_CUSTOM=y'
 config 'CONFIG_BUSYBOX_CONFIG_SHA256SUM=y'
+config 'CONFIG_BUSYBOX_CONFIG_SHA512SUM=y'
 config '# CONFIG_BUSYBOX_CONFIG_FEATURE_PREFER_IPV4_ADDRESS is not set'
 
 config 'CONFIG_PACKAGE_ATH_DEBUG=y'