nxp: Finish switch to CONFIG_NXP_ESBC

There are two remaining users of the CONFIG_SECURE_BOOT symbol that have
not been migrated to another symbol.  In this case, they should be using
CONFIG_NXP_ESBC as their guard.

Cc: Vladimir Oltean <olteanv@gmail.com>
Fixes: 5536c3c9 ("freescale/layerscape: Rename the config CONFIG_SECURE_BOOT name")
Signed-off-by: Tom Rini <trini@konsulko.com>