Changing parameter from `--cmd` switch on start

I am trying to figure out if this is a problem with MAVProxy or sim_vehicle.py but I think it is an issue with MAVProxy.
When MAVProxy gets a string for the `--cmd` switch it runs `module load` just fine but if you do something with the params it will say `Unable to find parameter 'PARAMETERNAME'`. Then if you run the same command once it has connected to the vehicle, it will work.
I have started a simulated instance with the output `udp:127.0.0.1:14560` and then ran
```
mavproxy.py --master=udp:127.0.0.1:14560 --cmd="param set SYSID_THISMAV 3"
```
and it says `Unable to find parameter 'SYSID_THISMAV'`. But if I run that command after it connects it is fine. So it seems there is an issue with how MAVProxy handles the `--cmd` switch on start.
Should it wait to run the commands it gets in the `--cmd` switch until it gets heartbeats?

Reproduce the issue with mavproxy.py using --master=udp:127.0.0.1:14560 and --cmd="param set SYSID_THISMAV 3" against the simulated instance. Inspect how startup --cmd commands are processed relative to the first heartbeat and parameter availability; done means the command succeeds at startup without reporting that the parameter cannot be found.

Written by the indexing model from the issue text.