kernel: init: mark initial dummy thread

The initial dummy thread context used for the initial __swap to
the main thread at early kernel initialization was not marked as a dummy
thread as it ought to be.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
diff --git a/kernel/init.c b/kernel/init.c
index 593e03f..cfed8b5 100644
--- a/kernel/init.c
+++ b/kernel/init.c
@@ -242,6 +242,7 @@
 	_current = dummy_thread;
 
 	dummy_thread->base.user_options = K_ESSENTIAL;
+	dummy_thread->base.thread_state = _THREAD_DUMMY;
 #endif
 
 	/* _kernel.ready_q is all zeroes */