atusb/fw/: disable INT0 if manipulating GPIOs (tentative)
atusb/fw/: disable INT0 if manipulating GPIOs (tentative)

Since GPIO manipulations may cause stray INT0 activity, we turn off
INT0 handing on ATUSB_GPIO. The MCU must be reset to restore access
to INT0.

There are still issues to resolve with the GPIO test. It may turn
out that there's a less invasive solution than just turning off
INT0 completely.

- board_app.c (gpio): mask INT0 before manipulating GPIOs
- board_app.c (board_app_init): make it clearer that EIMSK is a bit
  mask

cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan