configs: fsl-layerscape: secure_boot: Enable setexpr command

setexpr command is used while running secure boot
(chain of trust with confidentiality) feature.

Enable CONFIG_CMD_SETEXPR to enable setexpr command.

Signed-off-by: Udit Agarwal <udit.agarwal@nxp.com>
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>