rc.interface: allow setting a custom MIXER_FILE

ROMFS/px4fmu_common/init.d/rc.interface

@@ -128,17 +128,20 @@ then
 		set MIXER_AUX ${MIXER}
 	fi
 
-	if [ -f ${SDCARD_MIXERS_PATH}/${MIXER}.main.mix ]
+	if [ "$MIXER_FILE" == none ]
 	then
-		# Use the mixer file from the SD-card if it exists.
-		set MIXER_FILE ${SDCARD_MIXERS_PATH}/${MIXER}.main.mix
-	else
-		# Try out the old convention, for backward compatibility.
-		if [ -f ${SDCARD_MIXERS_PATH}/${MIXER}.mix ]
+		if [ -f ${SDCARD_MIXERS_PATH}/${MIXER}.main.mix ]
 		then
-			set MIXER_FILE ${SDCARD_MIXERS_PATH}/${MIXER}.mix
+			# Use the mixer file from the SD-card if it exists.
+			set MIXER_FILE ${SDCARD_MIXERS_PATH}/${MIXER}.main.mix
 		else
-			set MIXER_FILE /etc/mixers/${MIXER}.main.mix
+			# Try out the old convention, for backward compatibility.
+			if [ -f ${SDCARD_MIXERS_PATH}/${MIXER}.mix ]
+			then
+				set MIXER_FILE ${SDCARD_MIXERS_PATH}/${MIXER}.mix
+			else
+				set MIXER_FILE /etc/mixers/${MIXER}.main.mix
+			fi
 		fi
 	fi
 
@@ -168,7 +171,6 @@ then
 		tone_alarm ${TUNE_ERR}
 	fi
 
-	unset MIXER_FILE
 else
 	if [ $MIXER != skip ]
 	then

ROMFS/px4fmu_common/init.d/rcS

@@ -65,6 +65,7 @@ set MAVLINK_COMPANION_DEVICE    /dev/ttyS2
 set MAV_TYPE                    none
 set MIXER                       none
 set MIXER_AUX                   none
+set MIXER_FILE                  none
 set MK_MODE                     none
 set MKBLCTRL_ARG                ""
 set OUTPUT_MODE                 none
@@ -546,6 +547,7 @@ unset MAVLINK_COMPANION_DEVICE
 unset MAV_TYPE
 unset MIXER
 unset MIXER_AUX
+unset MIXER_FILE
 unset MK_MODE
 unset MKBLCTRL_ARG
 unset OUTPUT_DEV