DEVFS - Factor out recursive code into iterate_topology

DEVFS - Factor out recursive code into iterate_topology

* Factor out the recursive code which scans through all nodes in
  the topology into a function, devfs_iterate_topology, which takes
  a callback as argument, which is called for every node.

* Create new callback functions for all functions that were recursive
  before, containing the code that was executed for each node.

* Change all callers of recursive functions to call
  devfs_iterate_topology with the respective callback function
  instead of having their own recursive functions.