image: Add device tree setup to image library

image: Add device tree setup to image library

This seems to be a common function for several architectures, so create
a common function rather than duplicating the code in each arch.

Also make an attempt to avoid introducing #ifdefs in the new code, partly
by removing useless #ifdefs around function declarations in the image.h
header.

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