To build an object file from a device tree currently we currently
have to add them to the Makefile twice, once to dtb-y and once to
obj-y. This patch introduces obj-dtb-y and pbl-dtb-y to directly
compile a device tree object file for inclusion in the barebox binary
or the pbl respectively.
The now unneeded dtb-y targets are removed.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>