Merge pull request #422 from jean-m-cyr/master

Comments in GPS UORB topic don't make sense
Enabling navigator, does not do anything useful yet
Fix the direction of the override switch for the new state machine
easystar startup script: load mixer from sd-card if available
Merge pull request #427 from thomasgubler/sd_mixer

easystar startup script: load mixer from sd-card if available
Hotfix: Improved file test
Merge branch 'master' of github.com:PX4/Firmware
tone_alarm: add GPS warning tone
Merge branch 'master' of https://github.com/PX4/Firmware