Allow boards to initialize the DT at runtime.

In some boards like the Raspberry Pi the initial bootloader will pass
a DT to the kernel. When using U-Boot as such kernel, the board code in
U-Boot should be able to provide U-Boot with this, already assembled
device tree blob.

This patch introduces a new config option CONFIG_OF_BOARD to use instead
of CONFIG_OF_EMBED or CONFIG_OF_SEPARATE which will initialize the DT
from a board-specific funtion instead of bundling one with U-Boot or as
a separated file. This allows boards like the Raspberry Pi to reuse the
device tree passed from the bootcode.bin and start.elf firmware
files, including the run-time selected device tree overlays.

Signed-off-by: Alex Deymo <deymo@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/fdtdec.h

@@ -1023,4 +1023,10 @@ int fdtdec_setup_memory_banksize(void);
  */
 int fdtdec_setup(void);
 
+/**
+ * Board-specific FDT initialization. Returns the address to a device tree blob.
+ * Called when CONFIG_OF_BOARD is defined.
+ */
+void *board_fdt_blob_setup(void);
+
 #endif