arm64: ls1046ardb: Add distro secure boot support

Enable validation of boot.scr script prior to its execution dependent
on "secureboot" flag in environment. Disable fall back option to
qspi boot in case of secure boot. Also enable "secureboot=y" flag
in environment for ARM based platforms instead of bootcmd.

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