The Xenomai package directory includes a xenomai-do-not-install-devices.patch. It makes "install-exec-local" not depend on "devices". The same effect is implemented by the existing target "install-user".
I've submitted a patch to implement this suggestion, visible at http://lists.busybox.net/pipermail/buildroot/2013-October/081479.html.
Merged: 344fc7aa0d9879a051753b972a587aed59a180c2 xenomai: use install-user to remove patch