ARM: tegra: fix extlinux.conf search location

extlinux.conf is stored in /boot/extlinux/extlinux.conf rather than
/boot/extlinux.conf. Adjust Tegra's default boot scripts to use the
correct location. This change aligns Tegra's boot scripts with rpi_b.h
and also the location that the Fedora installer actually puts the file.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>