- removed ISR_STACKSIZE define where unused (set to 0)
- removed thread_arch_isr_stack_usage(), thread_arch_isr_stack_start(),
and/or thread_arch_isr_stack_pointer() where not implemented
If the watchdog is being reset in another thread, then
the reboot will never take place. This commit disables interrupts prior
to setting the watchdog timer to 250ms. This functionallity was common
to all Atmega CPUs, so it was moved to atmega_common.