tegra: Always build a boot image with the same filename

Adjust the Makefile to build u-boot-tegra.bin which contains a device tree
if OF_SEPARATE is enabled, and does not if not. This mirrors U-Boot's new
approach of using u-boot.bin to handle both cases.

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