Object : example of PCF8574 button proc driver

Test HW: Linkit 7688

Test SW: openwrit 3.18.109

Usage:

copy hello_btn to openwrt/package/kernel
make menuconfig
	select
	Kernel modules
	Other modules
	[*]kmod-hello_btn
make package/hello_eeprom/compile V=s
opkg install ipk to Linkit 7688

cat /proc/HELLO_BTN

Note:

The analog button seems simple. 
But, after 1~3 years, the button maybe be damped and there is some resistance.
One day, you want to press button to cancle launching missle. 
But,bad button launches the missile XD