[fix]flex_button_scan增加返回值，使得低功耗实现变得方便

[wzd5230]

1. fix error return type in flex_button_register

[update]

1. add return type of flex_button_process.
2. add return type of flex_button_scan. In low power system,the button task can get the return value when call flex_button_scan.Task stop polling when return 0,otherwise go on polling.In this case,the interrupt of button should be enabled,mcu will be waked up when get interrupt,and polling will restart.

Signed-off-by: wzd5230 772461552@qq.com

[murphyzhao]

That‘s great！