Only start tahoelafs if it's installed

src/freedombone-image-customise

     echo '' >> $START_DESKTOP
     echo '' >> $START_DESKTOP
     echo "if [ ! -f $FIRST_BOOT ]; then" >> $START_DESKTOP
-    echo "  $TAHOE_COMMAND start" >> $START_DESKTOP
+    echo '  if [ -d /var/lib/tahoelafs ]; then' >> $START_DESKTOP
+    echo "    $TAHOE_COMMAND start" >> $START_DESKTOP
+    echo '  fi' >> $START_DESKTOP
     echo '  exit 0' >> $START_DESKTOP
     echo 'fi' >> $START_DESKTOP
     echo '' >> $START_DESKTOP