README for Brillo GPIO sample

This GPIO sample demonstrates how to set up GPIO and write/read to GPIO pins.
Specifically this app does the following.

 - Export an input and output GPIO pins
 - Set output e.g. pin 24 direction to "out" and value to 1
 - Set input e.g. pin 28 direction to "in" and edge to "both"
 - Read input pin by polling
 - Once input pin is triggered to input value 1, set output value to 0

Once built, you can find the binary inside your tree at:

out/target/product/dragonboard/symbols/system/bin/gpio_playground

Do the following to deploy the sample:
 - adb root
 - adb remount
 - adb sync

Hardware setup:
 - You need to connect a GPIO pin as output to an LED
 - You need to connect a GPIO pin as input to a push button switch

Do the following to run the sample on Dragonboard and check logs for info:
 - Open a shell window and run: adb logcat
 - Open another shell window and run:
   - Default input pin 28 and output pin 24
     - adb shell '/system/bin/gpio_playground &'
   - Choose your own output and input pins
     - adb shell '/system/bin/gpio_playground --gin=<input> --gout=<output> &'
     - For example, adb shell '/system/bin/gpio_playground --gin=28 --gout=24 &'
 - Observe the logging info from the first window
 - Observe that the LED of pin 24 is turned on
 - Now push the switch of pin 28 and see that the LED gets turned off

Notes:
 - There is a GPIO port offset defined by const kDefaultGpioOffset that depends 
   on what kind of boards you use. For Dragonboard it is 902. This offset can
   also be configured by a commandline parameter.
 - There is an output pin and input pin defined by const kDefaultGpioOut and
   kDefaultGpioIn respectively.  They can both be configured by commandline
   parameters.