[kernel] Let endpoints get interrupt notifications

- Add a tag field to all endpoint messages, which doubles as a
  notification field
- Add a endpoint_bind_irq syscall to enable an endpoint to listen for
  interrupt notifications. This mechanism needs to change.
- Add a temporary copy of the serial port code to nulldrv, and let it
  take responsibility for COM2

modules.yaml

@@ -83,8 +83,10 @@ modules:
         deps:
           - libc
         source:
+            - src/drivers/nulldrv/io.cpp
             - src/drivers/nulldrv/main.cpp
             - src/drivers/nulldrv/main.s
+            - src/drivers/nulldrv/serial.cpp
 
     kutil:
         kind: lib