nanokernel: Add fiber_config structure and wrapper

Many drivers need to start fibers whose parameters are based on user
configuration.  Make it easier to specify such parameters by creating a
fiber_config structure and a wrapper to use it.

Change-Id: I26917ada71acbc676c0f32b6ee081aff7bb1d6d8
Signed-off-by: Vlad Dogaru <vlad.dogaru@intel.com>