tegra20: add u-boot-*-tegra.bin targets

tegra20: add u-boot-*-tegra.bin targets

Add target for tegra20 u-boot image.  This is a concatenation of tegra
spl and normal u-boot binaries.  For non-devicetree builds this is
named "u-boot-nodtb-tegra.bin" for devicetree builds is named
"u-boot-dtb-tegra.bin".

Signed-off-by: Allen Martin <amartin@nvidia.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Tested-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Tom Warren <twarren@nvidia.com>