sandbox: Allow ethernet to be disabled at runtime

For bootstd tests it is seldom useful to have ethernet enabled. Add a way
to disable it, so that ethernet operations like tftpboot do nothing.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/include/asm/test.h b/arch/sandbox/include/asm/test.h
index 568738c..1c522e3 100644
--- a/arch/sandbox/include/asm/test.h
+++ b/arch/sandbox/include/asm/test.h
@@ -344,4 +344,20 @@
  */
 int sandbox_load_other_fdt(void **fdtp, int *sizep);
 
+/**
+ * sandbox_set_eth_enable() - Enable / disable Ethernet
+ *
+ * Allows control of whether Ethernet packets are actually send/received
+ *
+ * @enable: true to enable Ethernet, false to disable
+ */
+void sandbox_set_eth_enable(bool enable);
+
+/**
+ * sandbox_eth_enabled() - Check if Ethernet is enabled
+ *
+ * Returns: true if Ethernet is enabled on sandbox, False if not
+ */
+bool sandbox_eth_enabled(void);
+
 #endif