thin: support chaining of external origins

thin: support chaining of external origins

Improve support for building DM tree when there is a chain
of external origins used for LV.

For this we cannot use track_external_lv_deps as this works
only for LV with just one external origin in its device tree.

Instead add directly 'dev' to the instead of add whole LV.
This avoid possibly recurive endless loop, however we may eventally
have some problems with undiscovered/missing devices in DM tree.