freescale/layerscape: Rename the config CONFIG_SECURE_BOOT name

Rename CONFIG_SECURE_BOOT to CONFIG_NXP_ESBC to avoid conflict
with UEFI secure boot.

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