console: Add a way to output to serial only

console: Add a way to output to serial only

In the video drivers it is useful to print errors while debugging but
doing so risks an infinite loop as the debugging info itself may go
through the video drivers.

Add a new console function that prints information only to the serial
device, thus making it safe for use in debugging.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>