Bug 25464

I was attempting to build a bridge using the above configuration.  Packets
were visible on either side of the bridge, but nothing would go across the
interfaces.  'netstat -p bdg' showed nothing but the header, and ifconfig of
the interface did not show the 'PROMISC' flag.

After some hours, I went back to the start and build a new kernel based on
GENERIC with just 'options BRIDGE' added.  This worked.  After attempting to
give myself a concussion on the desk, I compared the kernel configs line for
line and could only notice that the miibus and if_xl drivers were in-kernel
in the successful config and modular in the failing config.  Making the
drivers in-kernel in my original kernel config made bridging work.

Fix: 

Build the drivers into the kernel as a stopgap solution and document the
fact that this step is required.  Long term, figure out why the driver
doesn't work in this configuration.
How-To-Repeat: 
Build a bridging machine with the miibus and xl drivers but do not build
them into the kernel.  Reboot and attempt to pass packets over the bridge.