The xcvr base inherits from this (as does rx and tx base).
Added m4 macro to check for compiler flags.
The configure checks flags and headers.
Merged the register subdev static methods into one method
that associates a dboard id with a dboard constructor.
The manager code is responsible for checking this association
and creating the correct subdev instances.
Sub-classes only need to pass the single argument,
makes implementation cleaner.
Added a dboard registry and api for the manager.
This way, one could externally register custom dboard from the api.
