libusb: introduce jtag_libusb_choose_interface() and use it for JLink

libusb: introduce jtag_libusb_choose_interface() and use it for JLink

This introduces a new common function that allows auto-discovery of a
suitable USB interface based on class, subclass and protocol
matching. It claims the interface and returns the corresponding
endpoints number to the caller.

The need for this arised due to nRF51822 USB dongle which comes with
an "on-board Segger J-link debugger" having 3 interfaces, so the
current code can't work at all with it (in this particular case the
last interface needs to be choosen). This also removes special
handling of JLink-OB endpoint numbers as it's now possible to
autodetect them as well as the standard JLink endpoints.

Change-Id: I4d990a7a3b373efdd2949a394b32d855a168e138
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2327
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>