Microblaze: build kernel with device tree

This patch adds the options needed to build the SimpleImage containing
the device tree structure needed for the Microblaze architecture.

Handling Device Tree and SimpleImage will be handled in a general way
in the future.

I provide this patch to be able to build the system in the meantime.

Signed-off-by: Stephan Hoffmann <sho@relinux.de>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>