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>

net/net.c

@@ -106,6 +106,7 @@
 #endif
 #include <watchdog.h>
 #include <linux/compiler.h>
+#include <test/test.h>
 #include "arp.h"
 #include "bootp.h"
 #include "cdp.h"
@@ -465,6 +466,9 @@ restart:
 	debug_cond(DEBUG_INT_STATE, "--- net_loop Init\n");
 	net_init_loop();
 
+	if (!test_eth_enabled())
+		return 0;
+
 	switch (net_check_prereq(protocol)) {
 	case 1:
 		/* network not configured */