fdt: Build an SPL binary without device tree

At present u-boot-spl.bin holds the plain SPL binary without the device
tree. This is somewhat annoying since you need either u-boot-spl.bin or
u-boot-spl-dtb.bin depending on whether device tree is used.

Adjust the build such that u-boot-spl.bin includes a device tree
(if enabled), and the plain binary is in u-boot-spl-nodtb.bin. For now
u-boot-spl-dtb.bin remains the same.

Tested-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Simon Glass <sjg@chromium.org>