Move the stub logic to the device files themselves. This makes the
configure & build logic more static which will make it easier to move
to the top-level build, and matches what we did with the common/ hw
tree already.
This also decouples the logic from the two -- in the past, you needed
both sem & shm in order to enable the device models, but now each one
is tied to its own independent knob. Practically speaking, this will
probably not make a difference, but it simplifies the build a bit.
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