Kernel watchdog timeout

Implement kernel watchdog that detects when firmware becomes
unresponsive.

Change-Id: I5c5b58a56a2ce629e1fd7cabae83b61823239ea6
diff --git a/kernel/Kbuild b/kernel/Kbuild
index a6664ac..0b92c12 100644
--- a/kernel/Kbuild
+++ b/kernel/Kbuild
@@ -26,4 +26,5 @@
                ethosu_inference.o \
                ethosu_mailbox.o \
                ethosu_network.o \
-               ethosu_network_info.o
+               ethosu_network_info.o \
+               ethosu_watchdog.o