console: add console_flush_stdin()

Add a common helper console_flush_stdin() to drain all pending
characters from stdin. This consolidates the open-coded
while (tstc()) getchar() pattern that appeared in multiple places
across the tree.

Signed-off-by: Gregor Herburger <gregor.herburger@linutronix.de>
Reviewed-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/console.h

@@ -202,6 +202,11 @@ int console_clear(void);
  */
 int console_remove_by_name(const char *name);
 
+/**
+ * console_flush_stdin() - drops all pending characters from stdin
+ */
+void console_flush_stdin(void);
+
 /*
  * CONSOLE multiplexing.
  */