armv8: Secure Boot: Modify boot_a_script definition

esbc_validate command will not be executed if “load” command for its
header fails and will further execute the source command for bootscript,
without its validation and boot process continues.

To halt the  boot process in case secure boot header is not loaded
successfully, esbc_validate command is invoked separately after “load”
command. The secure boot validation of the bootscript header will fail
(if header is not loaded) and halts the boot process, which prevent source
command from execution.

Signed-off-by: Vinitha V Pillai <vinitha.pillai@nxp.com>
Signed-off-by: Udit Agarwal <udit.agarwal@nxp.com>
Reviewed-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>