Identify/fix PCM_bootloader.py failure at LAM

We had a PCM_bootloader failure at LAM, where the download failed and the PCM was not recoverable using PCM_bootloader.

Cause? It looked like the open() of the hexfile failed, but the program had already erased the PCM program image. xcuActor 1.10.2 checks the availability of the hex file earlier, so that should not happen again.

But we do not know why we could not recover the PCM. The vendor's DOS program which our python code is based on was able to recover the device over the network, so we know that the boot loader was still fine.

Our program was able to find the PCM on the network, so the strange multicast stuff at the start worked. But the command to set the IP address failed: the PCM never responded to it.

For a working device:

2019-06-17 13:43:09,774 forcing IP address to 10.1.12.4...
2019-06-17 13:43:09,775 send: b'5aa553990001010a010c04'
2019-06-17 13:43:10,664 recv: b'5aa5000000010101101453990a010c04040000'

In our case we did not get the response.

I'd tcpdump the DOS and our programs and look for diffs.