drivers/jtag: rewrite usb_blaster driver

Rewrite the Altera USB Blaster dongle driver :

 - make extensive use of byte-shift mode, to improve JTAG
   speed.
   This is the main reason of the rewrite. It improves the
   memory dumps with a factor 3 at least, and upload 100
   times, from 1 kBytes/sec to 100 kBytes/sec with a
   USB-Blaster connected to an Altera Virtual JTAG TAP +
   OpenRISC CPU.

 - split the low level API part (between FTDI and FTD2xx)
 from core driver, so that in the future, if both libftdi
 and ftd2xx can coexist, the driver will be able to switch
 dynamically from one access to the other.

Change-Id: I2ee9cedf4a5eb27501f337993ee0cdee52517e7c
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Marek Czerski <ma.czerski@gmail.com>
Tested-by: Franck Jullien <franck.jullien@gmail.com>
Reviewed-on: http://openocd.zylin.com/467
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Alexandre Becoulet <alexandre.becoulet@free.fr>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>