cmd/button: return button status

cmd/button: return button status

To make the button command useful in a shell script it should return the
status of the button:

* 0 (true) - pressed, on
* 1 (false) - not pressed, off

The button command takes only one argument. Correct maxargs.

Adjust the Python unit test.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Philippe Reynes <philippe.reynes@softathome.com>
[backport of mainline commit a6bfd71a96201127836d59736abcb54dc2d5e1a5]
Change-Id: Ic799b7e189e3fb6793083a92b2ceadb0f5884889
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>