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/generic/irq.h

@@ -3,9 +3,11 @@
 
 #include <stdint.h>
 
+typedef unsigned long irqstatus_t;
+
 void irq_disable(void);
 void irq_enable(void);
-uint8_t irq_save(void);
-void irq_restore(uint8_t flag);
+irqstatus_t irq_save(void);
+void irq_restore(irqstatus_t flag);
 
 #endif // irq.h