armv8: ls1012ardb: Add distro secure boot support

Enable validation of boot.scr script prior to its execution dependent
on "secureboot" flag in environment. Enable fall back option to
qspi boot in case of secure boot.

Signed-off-by: Sumit Garg <sumit.garg@nxp.com>
Signed-off-by: Vinitha Pillai <vinitha.pillai@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>