irq: Allow boards to define the return type of irq_save()

The AVR wants a uint8_t return type for irq_save(), but other
architectures will generally prefer int.  Allow the board to configure
the size of the flag by introducing an irqstatus_t typedef.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/avr/serial.c

@@ -89,7 +89,7 @@ console_pop_input(uint8_t len)
                     , needcopy - copied);
             copied = needcopy;
         }
-        uint8_t flag = irq_save();
+        irqstatus_t flag = irq_save();
         if (rpos != readb(&receive_pos)) {
             // Raced with irq handler - retry
             irq_restore(flag);