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>
78b5243182