Added support for Multi-Image files that contain a device tree

If a Multi-Image file contains a third image we try to use it as a
device tree.  The device tree image is assumed to be uncompressed in the
image file.  We automatically allocate space for the device tree in memory
and provide an 8k pad to allow more than a reasonable amount of growth.

Additionally, a device tree that was contained in flash will now automatically
get copied to system memory as part of boot.  Previously an error was
reported if one tried to boot a device tree that was in flash.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>