[ddk][serial] Move the serial-impl protocol to its own header file

TEST: zircon builds

Change-Id: I730ad51964b48ca7f28099d2bc67957a7a075f51

[zircon][ulib] Move usb out of driver ulib

This changeset moves usb.c out of the driver ulib and adds a separate
library for itself. In a later changeset some of the usb-request.c
functionality will move in into this library.

Test: Build. Sanity test of usb on vim2

Change-Id: I4807a3327943a71b5b88b22ab5c49756b042d969

[zircon][usb] Use usb protocol ops instead of usb-request functions

In a previous changeset, I added protocol ops for the usb request functionality.
The motivation behind the changeset is to move usb-request.c out of ddk
and support the operations in usb protocol instead. This changeset, uses those
protocol ops instead of calling the functions in usb-request.c. In a
later changeset, I will move the usb-request.c out of the ddk folder.

Test: Build. Sanity test on vim2

ZX-2404 #comment Start using protocol ops

Change-Id: Ib3f0d3929138a09b23704fa0ee80ccf62a121d35

[dev][serial][ftdi] Move ftdi driver to system/dev/serial/

system/dev/usb/ is just for drivers that implement the USB stack

Change-Id: I78fff13b2ba0b2028801419d8d0326e275664588