Introduce UCLASS-based extension board support to enable more
standardized and automatic loading of extension board device tree
overlays in preparation for integration with bootstd and pxe_utils.
Several #if CONFIG_IS_ENABLED are used in cmd/extension_board.c to ease the
development but don't worry they are removed later in the series.
Signed-off-by: Kory Maincent (TI.com) <kory.maincent@bootlin.com> Reviewed-by: Simon Glass <sjg@chromium.org>