Mantis: move SYS_DM_BACKEND to board_defaults

Otherwise it is set too late and not used during startup.

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

@@ -19,8 +19,6 @@
 set MIXER none
 set MIXER_AUX none
 
-param set-default SYS_DM_BACKEND 1
-
 # Battery settings
 param set-default BAT_CRIT_THR 0.20
 param set-default BAT_LOW_THR 0.25

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

@@ -18,6 +18,8 @@ param set-default SENS_MAG_MODE 0
 
 param set-default EV_TSK_STAT_DIS 1
 
+param set-default SYS_DM_BACKEND 1
+
 # Use MAVLink log streaming
 set LOGGER_ARGS "-m mavlink -c 0.5"