extension for touch panel fixture usage.

BUG=None
TEST=Manually on Beaglebone
  1. Check help string
     $ python poll_client.py -h

  2. Start polld
     $ python polld.py -d

  3. Check gpio7 is not exported yet
     $ ls -d /sys/class/gpio/gpio7 # should fail

  4. Run poll_client to poll gpio7
     $ python poll_client.py -r 100 gpio_falling:7

     Connect voltage 3.3V to GPIO7, then change to GND.
     The client waiting for GPIO7 should be notified.

  5. Run poll_client to read/write gpio7
     $ python poll_client.py -r 1 gpio_read:7
     --> gpio7 = 0
     $ python poll_client.py -r 1 gpio_set1:7
     $ python poll_client.py -r 1 gpio_read:7
     --> gpio7 = 1
     $ python poll_client.py -r 1 gpio_set0:7
     $ python poll_client.py -r 1 gpio_read:7
     --> gpio7 = 0

  6. Stop polld.py by Ctrl+C or SIGTERM.
     Check gpio7 under sysfs are unexported again.

Change-Id: I6e8aff6dd36a803a00bc180e21b7ba6e149d89b0
Reviewed-on: https://chromium-review.googlesource.com/266960


Tested-by: Pin-chih Lin <johnylin@chromium.org>
Reviewed-by: Pin-chih Lin <johnylin@chromium.org>
Commit-Queue: Pin-chih Lin <johnylin@chromium.org>