build: fix broken ftd2xx bus blaster

build: fix broken ftd2xx bus blaster

If configure is executed without --enable-ft2232_ftd2xx then the bus blaster
or presto will fail to build with unresolved external ftd2xx_status_string.

Make sure we run the ftd2xx build test if --enable-usb_blaster_ftd2xx is enabled.

Change-Id: I09d270d6fcd083d77f6785b8969d9acb3dfef11d
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/892
Tested-by: jenkins
Reviewed-by: Peter Stuge <peter@stuge.se>