fdt: Add DEV_TREE_BIN option to specify a device tree binary file

In some cases, an externally-built device tree binary is required to be
attached to U-Boot. An example is when using image signing, since in that
case the .dtb file must include the public keys.

Add a DEV_TREE_BIN option to the Makefile, and update the documentation.

Usage is something like:

	make DEV_TREE_BIN=boot/am335x-boneblack-pubkey.dtb

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