posix shell: lots of cleanup and fixes

- move posix-configs/SITL/init/{rcS,10016_iris,6011_typhoon_h480} to ROMFS/px4fmu_common/init.d-posix
  allows for easier unification, only one symlink is required.
  - rcS: add AUTOCNF support. Update scripts to match with behavior of PX4
    master (parameter values, some sitl driver got removed)
- add alias to allow 'set variable value' syntax in scripts to px4-alias.sh
- use px4_getopt
- use separate argument for the test_data directory
- append PATH from within the px4 binary: this simplifies the usage
  w/o the sitl_run.sh script.
- add 'source px4-alias.sh' to all existing sitl startup scripts
- move sitl mixers to ROMFS/px4fmu_common/mixers-sitl
  makes it easier to use existing mixers and sitl-specific ones.
- remove unused rcS_gazebo_delta_wing