Controller FPGA code cannot handle commands sent over multiple network packets

The command parser for the cobra FPGA does not wait for new TCP traffic to arrive and be queued in order to complete receiving a command of a known length. After the first (or some) packet has been consumed,  the parser sees no data and drops the command. 

In addition, it does not return an error packet (which are defined in the protocol).

I have a concise tcpdump file for this, which I'll annotate and attach.

At first glance, the fixes are pretty simple, though I think we should look at the (small) code a bit more carefully: there are other infelicities. Note that the code in question is in the C part of the image, not the FPGA part.

While doing this, I suggest we fix INSTRM-464 and add a version command.

The code is documented to have been built with Vivado 2017.2. That is still available, but newer versions are probably fine: Xilinx is in the business of stability.