fdt: Allow device tree to specify secure booting

When secure booting is chosen:
* The u-boot shell is never invoked during boot--we just do a simple
  table lookup to find the command.  This means we could even remove
  the shell parsing from u-boot and still be able to boot.
* The boot command can't be interruped.
* Failure doesn't cause us to fall back to the shell.

Signed-off-by: Gabe Black <gabeblack@google.com>
Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>