kbuild: Do not append dtb for OF_EMBED case

dtb is already included in binary that's why there is no need to replace
u-boot-spl.bin with u-boot-spl-dtb.bin. This is only needed for
OF_SEPARATE is enabled. Only copy -nodtb.bin version which is straight
output from objcopy -O binary.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>