bootstd: Include the device tree in the bootflow

bootstd: Include the device tree in the bootflow

Some bootmeths provide a way to load a device tree as well as the base
OS image. Add a way to store this in the bootflow. Update the
'bootflow info' command to show this information.

Note that the device tree is not allocated, but instead is stored at
an address provided by an environment variable. This may need to be
adjusted at some point, but for now it works well and fits in with the
existing distro-boot scripts.

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