This patch adds support for verifying a signed boot.scr. With this in place
it's possible for run-time Linux to update boot.scr to set different
variables such as switching between different boot partitions, pointing to
different kernels etc and for u-boot to verify these changes via the HAB
prior to executing the commands contained in boot.scr.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Tested-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>