fdt: Don't call board_fdt_blob_setup() without OF_BOARD

At present this override function is called even when OF_BOARD is not
enabled. This makes it impossible to disable this feature and in fact
makes the OF_BOARD option useless.

Reinstate its intended purpose, so that it is possible to switch between
the appended devicetree and one provided by the board's custom function.

A follower patch adds warnings for this scenario, but for now we don't
have a Kconfig that definitively tells us that OF_BOARD should be used.

Signed-off-by: Simon Glass <sjg@chromium.org>