serial: Implement serial_flush() function for console flush() fallback

Like in all other console functions, implement also serial_flush() function
as a fallback int console flush() function.

Flush support is available only when config option CONSOLE_FLUSH_SUPPORT is
enabled. So when it is disabled then provides just empty static inline
function serial_flush().

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/common/console.c b/common/console.c
index ce0c9b6..0c9bf66 100644
--- a/common/console.c
+++ b/common/console.c
@@ -797,6 +797,9 @@
 	if (gd->flags & GD_FLG_DEVINIT) {
 		/* Send to the standard output */
 		fflush(stdout);
+	} else {
+		/* Send directly to the handler */
+		serial_flush();
 	}
 }
 #endif