busybox: tweak IPv6/largefile handling

busybox: tweak IPv6/largefile handling

Fixes #8736

When IPv6 and largefile options were removed from Buildroot, the code to
force these options in busybox were still left in.

There's no strong reason to forcefully enable these options (only to disable
options if the system cannot support it like we do for nommu), so instead
enable the options in our default defconfig, allowing people to override
this if they use a custom config.

While we're at it, enable the prefer-ipv4 option so network applets like
ntpd doesn't fail when dual stacked hosts are resolved from a system without
IPv6 support enabled in the kernel.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>