Avoid a need of use different PX4 firmware branch to run FG multi-vehicle simulation

[kaklik]

The current Firmware master branch could be only used with more complicated multi-vehicle startup script.

The branch difference is in sitl_run.sh and sitl_run_multi.sh scripts.

The supposed best solution is a merge of sitl_run.sh and sitl_run_multi.sh into one script (probably sitl_run.sh) which checks the presence of the PX4_ID variable. The new sitl_run.sh script should decide its behavior according to the single or multi-vehicle simulation required. In the case the requirement of multi-vehicle simulation is detected automatically the special target __multi is not needed.