rc.interface: allow setting a custom MIXER_FILE

ROMFS/px4fmu_common/init.d/rc.interface

@@ -128,6 +128,8 @@ then
 		set MIXER_AUX ${MIXER}
 	fi
 
+	if [ "$MIXER_FILE" == none ]
+	then
 		if [ -f ${SDCARD_MIXERS_PATH}/${MIXER}.main.mix ]
 		then
 			# Use the mixer file from the SD-card if it exists.
@@ -141,6 +143,7 @@ then
 				set MIXER_FILE /etc/mixers/${MIXER}.main.mix
 			fi
 		fi
+	fi
 
 	if [ $OUTPUT_MODE == mkblctrl ]
 	then
@@ -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