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>

cmd/button.c

@@ -75,11 +75,11 @@ int do_button(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
 
 	ret = show_button_state(dev);
 
-	return 0;
+	return !ret;
 }
 
 U_BOOT_CMD(
-	button, 4, 1, do_button,
+	button, 2, 1, do_button,
 	"manage buttons",
 	"<button_label> \tGet button state\n"
 	"button list\t\tShow a list of buttons"