ROMFS: fix SYS_USE_IO==0

When SYS_USE_IO was disabled, px4io would not start and thus there was no
RC. SYS_USE_IO=0 is interesting on Quads to avoid the IO and reduce output
latency.

Tested on Pixhawk 1 and Pixhawk 4

ROMFS/px4fmu_common/init.d/rc.interface

@@ -125,6 +125,11 @@ then
 	set MIXER_AUX none
 fi
 
+if [ $USE_IO == no ]
+then
+	set MIXER_AUX none
+fi
+
 if [ $MIXER_AUX != none -a $AUX_MODE != none ]
 then
 	#