build/platform: allow doing a loose lookup_request (return None instead of Constraint...

build/platform: allow doing a loose lookup_request (return None instead of ConstraintError) and allow subname in lookup_request.

In the platforms, insead of doing:
self.lookup_request("eth_clocks").rx
we can now do:
self.lookup_request("eth_clocks:rx")

This allows some try/except simplifications on constraints.