Document and fix '${varname}' usage

ROMFS/px4fmu_common/init.d/rc.interface

@@ -2,6 +2,12 @@
 #
 # Script to configure control interface
 #
+#
+# NOTE: environment variable references:
+#  If the dollar sign ('$') is followed by a left bracket ('{') then the
+#  variable name is terminated with the right bracket character ('}').
+#  Otherwise, the variable name goes to the end of the argument.
+#
 
 set SDCARD_MIXERS_PATH /fs/microsd/etc/mixers
 
@@ -17,17 +23,17 @@ then
 	fi
 
 	# Use the mixer file from the SD-card if it exists
-	if [ -f $SDCARD_MIXERS_PATH/$MIXER.main.mix ]
+	if [ -f ${SDCARD_MIXERS_PATH}/${MIXER}.main.mix ]
 	then
-		set MIXER_FILE $SDCARD_MIXERS_PATH/$MIXER.main.mix
+		set MIXER_FILE ${SDCARD_MIXERS_PATH}/${MIXER}.main.mix
 	# Try out the old convention, for backward compatibility
 	else
 
-		if [ -f $SDCARD_MIXERS_PATH/$MIXER.mix ]
+		if [ -f ${SDCARD_MIXERS_PATH}/${MIXER}.mix ]
 		then
-			set MIXER_FILE $SDCARD_MIXERS_PATH/$MIXER.mix
+			set MIXER_FILE ${SDCARD_MIXERS_PATH}/${MIXER}.mix
 		else
-			set MIXER_FILE /etc/mixers/$MIXER.main.mix
+			set MIXER_FILE /etc/mixers/${MIXER}.main.mix
 		fi
 	fi
 
@@ -133,14 +139,14 @@ then
 	set MIXER_AUX_FILE none
 	set OUTPUT_AUX_DEV /dev/pwm_output1
 
-	if [ -f $SDCARD_MIXERS_PATH/$MIXER_AUX.aux.mix ]
+	if [ -f ${SDCARD_MIXERS_PATH}/${MIXER_AUX}.aux.mix ]
 	then
-		set MIXER_AUX_FILE $SDCARD_MIXERS_PATH/$MIXER_AUX.aux.mix
+		set MIXER_AUX_FILE ${SDCARD_MIXERS_PATH}/${MIXER_AUX}.aux.mix
 	else
 
-		if [ -f /etc/mixers/$MIXER_AUX.aux.mix ]
+		if [ -f /etc/mixers/${MIXER_AUX}.aux.mix ]
 		then
-			set MIXER_AUX_FILE /etc/mixers/$MIXER_AUX.aux.mix
+			set MIXER_AUX_FILE /etc/mixers/${MIXER_AUX}.aux.mix
 		fi
 	fi