Add the trex ethermedium.

The intent is that for bleeding edge devices, we want our own medium
to hack on as we please. Anticipating some weird usage, I've added
the trexconfig script, which you can hack as you wish.

If you want to change basic functions in the medium, copy them and convert
them. I'm still not sure we don't want trexmedium.c but for now we decided not
to create a new file.

Test:
comment out the ethermedium struct (i.e. leave only the trexmedium available)
boot, run trexconfig, note network works.

uncomment out ethermedium, boot, run /trexconfig, it all works.
reboot, run ifconfig, it all still works.

I decided to skip the branch because this change is pretty harmless.

Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>