drivers: ethernet: build as static library

Instead of putting object files inside libzephyr.a,
simply build a separate static library as most other
driver types are doing this already.

Note that the include to subsys has been moved from
under the drivers into subsys, as it is actually
the subsystem's job to make sure the include
directories are correct.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>