fdt: Show a runtime warning based on devicetree source

When running, if the devicetree failed to come from the expected source,
show a warning, e.g:

   U-Boot ...

   DRAM:  128 MiB
   Core:  42 devices, 11 uclasses, devicetree: separate
   Warning: Unexpected devicetree source (not from a prior stage)
   Warning: U-Boot may not function properly
   Flash: 64 MiB
   ...

These warnings should only appear if the board config has been changed, or
the prior stage is broken.

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