Create spec for "detachable devices"

[aacuevas]

There are some devices whose HDL controller lives in the hub but the device itself can be connected or not. Examples are: The BNOs in the acq board. or the Neuropixel probes in a Neuropixel headstage.

After some discussion we agreed that the best way for these cases is for this state to be reflected on the device map. When performing a device map scan, if these devices are not present, even if the HDL block driving them is, these HDL devices will return a NULL device instead, to represent the lack of the actual represented device.