sxmo_brightness.sh: write brightness val directly to sysfs

This removes the dependency on an external compiled app for writing the
brightness value to sysfs, and just echoes it directly to the backlight
sysfs node.

In addition, if the write fails, the script exits instead of sending a
misleading notification.

Signed-off-by: Stacy Harper <contact@stacyharper.net>

scripts/core/sxmo_brightness.sh

@@ -17,12 +17,14 @@ MINSTEP=1
 STEP=$(echo "($MAX - $MIN) / 10" | bc | xargs -ISTP echo -e "$MINSTEP\nSTP" | sort -r | head -n1)
 
 setdelta() {
-	sxmo_setpinebacklight "$(
+	VAL="$(
 		xargs -IB echo B "$1" < $BACKLIGHT/brightness |
 		bc |
 		xargs -INUM echo -e "$MIN\nNUM" | sort -n | tail -n1 |
 		xargs -INUM echo -e "$MAX\nNUM" | sort -n | head -n1
 	)"
+	echo "$VAL" > "$BACKLIGHT/brightness" || exit 1
+	echo "set brightness to $VAL"
 
 	dunstify -i 0 -u normal -r 999 "☀ $(cat $BACKLIGHT/brightness)/${MAX}"
 }