[hpsdr] 122.88MHz coherence with two Mercuries

[Joe Martin K5SO]

Thanks Alfred,  for that!

I've already posted an updated (equivalently modified) version of  
Mercury v6.7.rbf, Quartus II archive file, and USBBlaster-Binaries to  
my website so that there is an option to use LVDS between the Mercs if  
you wish to share the 122.88 MHz clock  that way.

Alfred's observation that when using LVDS to distribute the 122.88MHz  
clock the second Merc doesn't start until the first is running is a  
potentially important one as the sequential LVDS method of sharing the  
122.88MHz clock has the possibility of stabilizing the relative phase  
between the multiple Mercs on start up (or not, hihi).  I will be  
checking that with my fingers crossed!

73,  Joe K5SO