Update the common Tegra boot scripts in the default environment to
a) Make use of the new "test -e" shell command to avoid some error
messages.
b) Allow booting using the sysboot command and extlinux.conf. This
allows easy creation of boot menus, and provides a simple interface
for distros to parameterize/configure the boot process.
Signed-off-by: Stephen Warren <swarren@nvidia.com> Signed-off-by: Tom Warren <twarren@nvidia.com>