Move setting of SPL_UBOOT_KEY_HASH to a non-NULL value to Kconfig. As part of this, change fsl_secboot_validate(...) to check that it is passed a non-empty string, rather than non-NULL. Cc: Peng Fan <peng.fan@nxp.com> Cc: Priyanka Jain <priyanka.jain@nxp.com> Cc: Kshitiz Varshney <kshitiz.varshney@nxp.com> Signed-off-by: Tom Rini <trini@konsulko.com>