[fault] Generalise NETDEV_DISCARD_RATE fault injection mechanism

Provide a generic inject_fault() function that can be used to inject
random faults with configurable probabilities.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/fault.h

@@ -0,0 +1,19 @@
+#ifndef CONFIG_FAULT_H
+#define CONFIG_FAULT_H
+
+/** @file
+ *
+ * Fault injection
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <config/defaults.h>
+
+/* Drop every N transmitted or received network packets */
+#define	NETDEV_DISCARD_RATE 0
+
+#include <config/local/fault.h>
+
+#endif /* CONFIG_FAULT_H */