openocd: add support for libftdi 1.5

openocd: add support for libftdi 1.5

The new libftdi 1.5 (2020-07-07) changes some API, deprecating the
old ones. This cause a warning at compile time.

Detect in configure the version of libftdi.
Use the new API in the driver's code.
Add an helper include file 'libftdi_helper.h' that wraps the old
API for backward compatibility with old libftdi.

Change-Id: I7800fbebe17dd0ce62e55b3598d8c08be8875bb7
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: https://sourceforge.net/p/openocd/tickets/286/
Reviewed-on: http://openocd.zylin.com/5891
Tested-by: jenkins