Add MPSSE communications layer for FTDI chips

This is a higher-level libftdi replacement for use when implementing
protocol drivers for FT2232, FT2232H or FT4232H. It takes care of device
open/close and, unlike libftdi, also MPSSE command abstraction, command
queueing, buffer handling and return data parsing.

The FTDI device is accessed through libusb-1.0 in asynchronous mode.

Change-Id: I051adb574dcc39f8ca9cd7f6dbe6ae4aeea5f4c8
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/451


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-by: Peter Stuge <peter@stuge.se>