* Move platform specific things (e.g. firewalling and route
  collision checks) into bridge_driver_platform
* Create two platform specific implementations:
    - bridge_driver_linux: Linux implementation using iptables,
      it's actually the code moved from bridge_driver.c
    - bridge_driver_nop: dumb implementation that does nothing
Signed-off-by: NEric Blake <eblake@redhat.com>