lbu: fix the way we check if a dir is mounted or not

this works around an issue when lbu wrongly assumes /media/usb is
mounted when /media/usbdisk is.

lbu.in

@@ -65,7 +65,7 @@ exit_clean() {
 }
 
 mount_once() {
-	if ! grep $1 /proc/mounts >/dev/null; then
+	if awk "\$2 == \"$1\" {exit 1}" /proc/mounts; then
 		mount $1 && UMOUNT_LIST="$1 $UMOUNT_LIST"
 	fi
 }