[rotorcraft] navigation: use NAV_FREQ to update stage and block time
[maps] hopefully finally fix google maps version download

seems like #865 didn't work with sed version on OSX after all...
So just use gnu sed on osx and the documented gnu sed -r option instead of -E

Also add a check if the extraced google version is actually a valid integer number..
[modules] dc: Some correction in description text
[rotorcraft] get rid of some warnings in ardrone2_sdk