Clarify LLDP MAC addresses used, improve documentation

The newly added EIP LDDP subsystem uses the port MAC addresses from the DDP/FDL (index 1 & 2) for at least some of its frames.

When the user sets only the first (EIP) and fourth (NDIS) MAC in the DDP, and the other to zero, the CTT18 will fail as reported with the linked ticket. The solution for now was to set MAC0 (first chassis) also into MAC1 and MAC2 (port macs). This should be feasible according to the spec. Though, it may also be considered harmful to have multiple equal MACs in the DDP for future maintenance and development.

The 2PS doesn't do much with the port MACs, I doubt the switch can even be addressed using them, but I have not checked it yet. So it is questionable if the EIP LLDP should reflect these port MACs at all. This shall be discussed and checked against the spec. Maybe we to interface the LDDP so that it only reflects the station MAC?

If we stay with the current approach of relying on  MAC1 and MAC2 in the DDP, then it must be covered in the API manual and the demo code.

Conclusion: We document in the API manual and demos projects that the 2nd and 3rd MAC address in the DDP should be set to valid port MAC addresses (just as Profinet does it). Typically, these are just an increment of the 1st MAC in the DDP.