Mantis: fix mixer loading

The ordering before did not work out, and without the sleep it fails.

ROMFS/px4fmu_common/init.d/airframes/4061_atl_mantis_edu

@@ -170,9 +170,6 @@ param set-default SENS_FLOW_MINHGT 0.15
 param set-default SENS_FLOW_MAXHGT 5.0
 param set-default SENS_FLOW_ROT    4
 
-
-mixer load /dev/tap_esc /etc/mixers/quad_x.main.mix
-
 # ignore the SD card errors and use normal startup sound, and make it less loud
 set STARTUP_TUNE "1"
 set STARTUP_TUNE_VOLUME "-s 20"

boards/atl/mantis-edu/init/rc.board_extras

@@ -1,6 +1,8 @@
 #!/bin/sh
 
 tap_esc start -d /dev/ttyS4 -n 4
+sleep 1
+mixer load /dev/tap_esc /etc/mixers/quad_x.main.mix
 
 # Transition from previous non open-source flight stack
 if param compare SYS_AUTOSTART 4001