Separate electrically JTAG signals from AMC conn and onboard header
Problem manifests itself when there's a JSM (JTAG Switch Module) present
in µTCA crate. In that case, neither JSM nor Xilinx JTAG will work.
I think the line of reasoning here should be: if external JTAG is
plugged into header, disconnect AMC JTAG, otherwise use AMC (and JSM).
With a simple circuit this switching could be done purely in hardware.
This is a bit connected with #1434.
Final archictecture could look like this: SCANSTA would switch between
AMC JTAG and connector JTAG, set by the presence of the connector,
probably pulling it down through its GND.
There could be two connectors footprints in the board, so we could use
Xilinx JTAG or LPCLink JTAG without modifications, but the jtag signals
would be shorted in the PCB.