This patch adds an "Ethernet Gadget" driver, implementing
the CDC Ethernet model (drivers/usb/gadget/ether.c).

It interops with the current CDC Ether drivers on Linux,
both 2.4 (CDCEther, using Marcelo's latest) and 2.5
(cdc-ether with recent patches, or on 2.5.68 "usbnet")

On a net2280, this has successfully streamed dozens of
megabytes per second using "ttcp" (high speed, and using
"usbnet" on the host side), for days at a time.  And no
problems using SSH/NFS/etc in lighter duty testing.

It's possible this will need tweaking to cope with UDC
bugs on Intel's pxa25x controllers, presenting itself
as a non-CDC device. (I'm told altsettings are even
more broken than originally specified to be.)