fdt: Allow the devicetree to come from a bloblist

Standard passage provides for a bloblist to be passed from one firmware
phase to the next. That can be used to pass the devicetree along as well.
Add an option to support this.

Tests for this will be added as part of the Universal Payload work.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

doc/develop/devicetree/control.rst

@@ -108,6 +108,9 @@ If CONFIG_OF_BOARD is defined, a board-specific routine will provide the
 devicetree at runtime, for example if an earlier bootloader stage creates
 it and passes it to U-Boot.
 
+If CONFIG_BLOBLIST is defined, the devicetree may come from a bloblist passed
+from a previous stage, if present.
+
 If CONFIG_SANDBOX is defined, then it will be read from a file on
 startup. Use the -d flag to U-Boot to specify the file to read, -D for the
 default and -T for the test devicetree, used to run sandbox unit tests.