Makefile: seperate out dtb building steps
Makefile: seperate out dtb building steps

We currently produce the .dtb.o.tmp and then convert that into the .dtb.o
file in the same rule.

Signed-off-by: Joel Stanley <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@jms.id.au>

open-power/pdbg,open-power/pdbg,open-power/pdbg