Buttons: Change return value of board_buttons() and the type of btn_buttonset_t to uint32_t so that more than 8 buttons can be supported.

configs/twr-k60n512/src/k60_buttons.c

@@ -94,9 +94,9 @@ void board_button_initialize(void)
  * Name: board_buttons
  ****************************************************************************/
 
-uint8_t board_buttons(void)
+uint32_t board_buttons(void)
 {
-  uint8_t ret = 0;
+  uint32_t ret = 0;
 
   if (kinetis_gpioread(GPIO_SW1))
     {