arm-bsp-extras/u-boot/fvp-rd-kronos: Add UEFI Secure Boot support

Modify U-Boot kronos.cfg file to enable UEFI Secure Boot.

Modify boot.cmd to enroll the authenticated variables
for UEFI Secure Boot.

Modify fvp-rd-kronos.conf file to add the generated
authenticated variables to the EFI boot files
to be deployed to the boot partition.

Modify u-boot-fvp-rd-kronos.inc file to pass authenticated
variables folder name to the boot.cmd file.

Signed-off-by: Amr Mohamed <amr.mohamed@arm.com>