Hello world!
Searching the forum for "mkfontdir" doesn't yield any results so I suppose nobody has noticed this yet.
Building fonts or Xorg may lead to problems. For building fonts the tools mkfontdir and mkfontscale are required; not for the target-arch, the host building openwrt needs them. If these tools are not installed, openwrt does not provide them. Usually these tools come as a dependency of the Xserver. No Xserver, no tools, complaining openwrt.
Since openwrt usually doesn't come with Xorg a the tools are required only for building this propably doesn't qualify as a bug, but maybe this post helps other people.
Cheers