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/cpu/state.c b/arch/sandbox/cpu/state.c
index dd7978c..267f280 100644
--- a/arch/sandbox/cpu/state.c
+++ b/arch/sandbox/cpu/state.c
@@ -12,6 +12,7 @@
 #include <os.h>
 #include <asm/malloc.h>
 #include <asm/state.h>
+#include <asm/test.h>
 
 /* Main state record for the sandbox */
 static struct sandbox_state main_state;
@@ -366,6 +367,7 @@
 	state->sysreset_allowed[SYSRESET_POWER_OFF] = true;
 	state->sysreset_allowed[SYSRESET_COLD] = true;
 	state->allow_memio = false;
+	sandbox_set_eth_enable(true);
 
 	memset(&state->wdt, '\0', sizeof(state->wdt));
 	memset(state->spi, '\0', sizeof(state->spi));
@@ -444,6 +446,20 @@
 	return 0;
 }
 
+void sandbox_set_eth_enable(bool enable)
+{
+	struct sandbox_state *state = state_get_current();
+
+	state->disable_eth = !enable;
+}
+
+bool sandbox_eth_enabled(void)
+{
+	struct sandbox_state *state = state_get_current();
+
+	return !state->disable_eth;
+}
+
 int state_init(void)
 {
 	state = &main_state;