Add a flag indicating when the serial console is ready

For sandbox we have a fallback console which is used very early in
U-Boot, before serial drivers are available. Rather than try to guess
when to switch to the real console, add a flag so we can be sure. This
makes sure that sandbox can always output a panic() message, for example,
and avoids silent failure (which is very annoying in sandbox).

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/console.c b/common/console.c
index 5576dfd..898da39 100644
--- a/common/console.c
+++ b/common/console.c
@@ -417,7 +417,7 @@
 void putc(const char c)
 {
 #ifdef CONFIG_SANDBOX
-	if (!gd) {
+	if (!gd || !(gd->flags & GD_FLG_SERIAL_READY)) {
 		os_putc(c);
 		return;
 	}
@@ -447,7 +447,7 @@
 void puts(const char *s)
 {
 #ifdef CONFIG_SANDBOX
-	if (!gd) {
+	if (!gd || !(gd->flags & GD_FLG_SERIAL_READY)) {
 		os_puts(s);
 		return;
 	}