riscv: Fix BUILTIN_DTB for sifive and microchip soc

Fix BUILTIN_DTB config which resulted in a dtb that was actually not
built into the Linux image: in the same manner as Canaan soc does,
create an object file from the dtb file that will get linked into the
Linux image.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>