Add board_pre_console_putc to deal with early console output

This patch adds support for console output before the console is inited.
The main purpose of this is to deal with a very early panic() which would
otherwise cause a silent hang.

A new board_pre_console_putc() function is added to the board API. If
provided by the board it will be called in the event of console output
before the console is ready. This function should turn on all UARTs and
spray the character out if it possibly can.

The feature is controlled by a new CONFIG_PRE_CONSOLE_PUTC option.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Graeme Russ <graeme.russ@gmail.com>