OK, this should be fixed now at bsd.own.mk 1.1378. the problem was that for the xen kernel builds, MACHINE=xen, and the bsd.own.mk snippet was checking for "i386" and "amd64", and not "xen". i was able to trigger the failure in a tree using an empty mk.conf, still not sure why i didn't see it in my normal trees.
.mrg.
