Make shutdown functions callable by OS image's exec methods.

src/include/gpxe/shutdown.h

@@ -0,0 +1,10 @@
+#ifndef _GPXE_SHUTDOWN_H
+#define _GPXE_SHUTDOWN_H
+
+/**
+ * Shut down before exit
+ */
+
+extern void shutdown ( void );
+
+#endif /* _GPXE_SHUTDOWN_H */