tegra: do not descend into empty directories

Some tegra makefiles only contain a dummy line to generate
a built-in.o.  Let's do not descend into such directories.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Tom Warren <twarren@nvidia.com>
Cc: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>