console: Add option to keep it silent until env is loaded

Add a config-option which forces the console to stay silent until the
proper environment is loaded from flash.

This is important when the default environment does not silence the
console but no output must be printed when 'silent' is set in the flash
environment.

After the environment from flash is loaded, the console will be
silenced/unsilenced depending on it.  If PRE_CONSOLE_BUFFER is also
used, the buffer will now be flushed if the console should not be
silenced.

Signed-off-by: Harald Seiler <hws@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>