ftdi: allow selecting device by usb bus location

ftdi: allow selecting device by usb bus location

This patch adds a 'ftdi_location' command to select an adapter by usb
bus number and port path.

This is helpful if you have a rack full of adapters in a testing or
manufacturing setup where the only constant is the physical usb bus
location of the adapter you want to address. Vid:Pid are not unique,
serial number _may_ be unique (and maybe not with embedded adapters) but
will change when a new target is plugged.

Specifying a location allows to understand instantly which board failed
bringup or testing.

Change-Id: I403c7c6c8e34fe42041b3f967db80f3160a4f1a3
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/3351
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>