event: add new EVT_SETTINGS_R event

Introduce EVT_SETTINGS_R, triggered post-relocation and before console
init.

This event gives an option to perform any platform-dependent setup,
which needs to take place before show_board_info(). Usage examples
include readout of EEPROM stored settings.

Signed-off-by: Artur Rojek <artur@conclusive.pl>
Reviewed-by: Simon Glass <sjg@chromium.org>