Support hla_serial command for ST-LINK adapters.

The hla_serial command allows for a programming device serial number to be
specified in addition to USB VID/PID.  This allows for multiple ST-LINK/V2
programmers to be attached to a single machine and operated using openocd.

Change-Id: I350654bf676eb26ba3a90450acfa55d2a5d2d791
Signed-off-by: Austin Phillips <austin_phillips@hotmail.com>
Reviewed-on: http://openocd.zylin.com/2198
Tested-by: jenkins
Reviewed-by: Martin Glunz <mg@wunderkis.de>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>