updated the build instructions for the hdmidisplay example

Change-Id: I225ed3de10979bc1e13992bbcc24194692b0c00f
xadc pins are LVCMOS25 on zc702, according to old version of xpsgen.py

Change-Id: I0627295548e7a387ae83b4c3ffda9b7b3db9dc0c
added hdmi pin constraints for hdmidisplay example

Change-Id: If20574d0b6e1592059a7df835101acab87216966
more robust test for TOP being defined

Change-Id: I941b72f0a6d831d91acbda8c4961799f4adebb98