The current ISR implementation for AVR8 requires use of
avr8_[enter/exit]_isr pair which add some boilerplate on code.
This add AVR8_ISR which clean-up the code and make it simpler
and hides any schedule detail from the user perspective.
This is a preparation for future scheduling and irq optimizations.
Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>
