rump into share/mk. This is to make it useful for all kernel
builders.
Note: we have waaay too many weird and wonderful ways of making
kernel code (monolithic kernel, modules, rump). There should be
only one way to build kernel code instead of a maze of twisty little
.mk files, all not quite alike. When that is fixed, this snippet can go
into the more generic .mk file.
pooka