Reported by Montiainen on IRC: in the nfs-utils package, the rpcdebug tool is built for the host, but installed on the target. This tool should probably be built for the target. The patch package/nfs-utils/0002-Patch-taken-from-Gentoo.patch is incorrect. Upstream has fixed the issue in a newer version of nfs-utils, see http://git.linux-nfs.org/?p=steved/nfs-utils.git;a=commit;h=adabe938c17b22aec5e89c24e1856de9fffe9f0c.
Fixed by the latest bump of nfs-utils, see http://git.buildroot.net/buildroot/commit/package/nfs-utils?id=e5e31fe92dd191480341d07a01f79f88abab9c2a.