ARM: tegra: rework boot scripts

Update the common Tegra boot scripts in the default environment to

a) Make use of the new "exists" shell command to avoid some error
   messges.

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>
pxe: implement fdtdir extlinux.conf option

FIXME: describe this

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