fsl_validate: Migrate SPL_UBOOT_KEY_HASH to Kconfig

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>