boards/zynq: rename the DTB to be loaded by U-Boot

boards/zynq: rename the DTB to be loaded by U-Boot

We are about to introduce genimage support for the Zynq boards. For
all of them U-Boot loads the DTB from a file named "devicetree.dtb" so
we need to rename it on the SD card image.

We could use genimage itself to rename files while creating the
image. But this would require a different genimage.cfg for each board,
leading to duplicate code.

Instead let's create a "devicetree.dtb" symlink pointing to the dtb
listed in the configuration. Currently all the Zynq-based boards have
only one DTS configured, but if one had two or more of them, the
symlink would point to the first one.

[Peter: only create symlink if dtb option is available]
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>