GESTURES.TXT

If a file called GESTURES.TXT exists in /boot, then the gesture service
is run.  This detects pushbutton gestures on a switch wired to GPIO 17, 18.

The switch is e.g. this one:  https://www.adafruit.com/product/559

and it is wired like so:

"+"         -> GPIO 17
"-", COMMON -> GND  (two terminals going to ground)
"NO"        -> GPIO 18 (normally-open terminal goes to GPIO 18)

Device-tree fragments are loaded to ensure the internal pull-up
resistor is enabled on GPIO 18 etc. so that grounding GPIO 18 is
considered "on".  Note that the adafruit switch's LED includes an
internal resistor to limit current.  If you roll your own LED, make
sure to do something similar.

The following gestures are supported:

single click: toggle a 1/s heartbeat of the LED as well as rapid
              blinks for detected tag pulses.  Turns off automatically
              after 10 minutes

double click: toggle the WiFi hotspot (RPi 3 only); the PI 3's
              internal WiFi adapter will turn on and create a
              WPA2-protected access point with the SG serial number
              as both essid and passphrase, e.g. SG-26F1RPI358CC The
              PI3 has the address 192.168.7.2 when connecting
              wirelessly.  When the WiFi is on, the LED blinks on and
              off every 0.9 seconds.

              Turns off automatically after 30 minutes.

hold 3 sec: perform a clean shutdown; the LED will light and stay
              lit until shutdown is complete, and then turns off.

See /home/pi/proj/sensorgnome/master/gestures.js and pushbtn.js for details.

In particular, when GESTURES.TXT exists, you will need to turn on the WiFi
hotspot using a double-clik of the switch mentioned above.

To have the WiFi hotspot on at the start of every boot session,
delete GESTURES.TXT

If you delete or rename this file, no gesture service is run on the next
boot.