buildroot does not currently support building against rc versions of kernel headers. The download step fails because the download directory differs between rc and final kernel releases. This patch tries to detect when an rc kernel is being used and download from the correct location.
Will, are you still interested by this? You never attached any patch to this bug. Since the bug is three years old and I know you've been active in Buildroot since that time, I suppose you're no longer that much interested in this feature, so I'm marking this as "won't fix". However, if you're interested in reviving this, do not hesitate to send a patch (I believe if we need such a feature, we should simply allow to specify a custom tarball location of kernel sources for the kernel headers).