external trigger for USBDUX

Hi guys,

there was somebody on this mailing list who wanted to be the guinnea pig 
for the TRIG_EXT for the dux board. I cannot find his e-mail. So, can 
you please get back to me.

Anyway. Here's the code. It should work with 1 channel and 16 channels. 
With 2 and 3 it's not possible because the FX2 has not enough states in 
the GPIF.

The board will wait for a positive flank on the trigger input. I've 
tried it with cmd.c and it works fine. Also fixed a bug in the 16 
channel mode.

/Bernd
-- 
www:    http://www.berndporr.me.uk/
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