fdt: Add a Kconfig for boards with a prior stage

When U-Boot is started from another firmware program, not just a prior
phase of U-Boot, special behaviour is typically used. In particular, the
device tree may come from that prior stage.

At present this is sort-of indicated by OF_BOARD, although the
correlation is not 1:1, since that option simply means that the board has
a custom mechanism for obtaining the device tree. For example, sandbox
defines OF_BOARD. Also the board_fdt_blob_setup() function can in fact
make use of the devicetree in U-Boot if it wishes, as used by
dragonboard410c until very recently.

Add an explicit Kconfig for this situation. Update the OF_BOARD option to
more-accurately reflect what it is doing, e.g. for sandbox.

Drop the docs in the README as it is out of date.

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