tunes: add tune to power off

This is used by the ATL Mantis EDU to help the user realize when the
drone is powered off.

msg/tune_control.msg

@@ -22,7 +22,8 @@ uint8 TUNE_ID_SD_ERROR             = 15
 uint8 TUNE_ID_PROG_PX4IO           = 16
 uint8 TUNE_ID_PROG_PX4IO_OK        = 17
 uint8 TUNE_ID_PROG_PX4IO_ERR       = 18
-uint8 NUMBER_OF_TUNES              = 19
+uint8 TUNE_ID_POWER_OFF            = 19
+uint8 NUMBER_OF_TUNES              = 20
 
 uint8 tune_id        # tune_id corresponding to TuneID::* from the tune_defaults.h in the tunes library
 bool tune_override   # if true the tune which is playing will be stopped and the new started

src/lib/tunes/tune_definition.desc

@@ -104,3 +104,4 @@ PX4_DEFINE_TUNE(15, SD_ERROR,              "MNBG",
 PX4_DEFINE_TUNE(16, PROG_PX4IO,            "MLL32CP8MB",                                    false /*  Program PX4IO */)
 PX4_DEFINE_TUNE(17, PROG_PX4IO_OK,         "MLL8CDE",                                       false /*  Program PX4IO success */)
 PX4_DEFINE_TUNE(18, PROG_PX4IO_ERR,        "ML<<CP4CP4CP4CP4CP4",                           true  /*  Program PX4IO fail */)
+PX4_DEFINE_TUNE(19, POWER_OFF,             "MFT255a8g8f8e8c8<b8a8g4",                       true  /*  When pressing off button */)