mx7dsabresd: Remove the mx7dsabresd_secure_defconfig target

mx7dsabresd: Remove the mx7dsabresd_secure_defconfig target

mx7dsabresd_secure_defconfig was introduced to allow booting NXP kernel
that has CAAM support and needs to boot in secure mode.

Instead of keeping two different config targets for the same board,
remove mx7dsabresd_secure_defconfig and select
CONFIG_ARMV7_BOOT_SEC_DEFAULT inside mx7dsabresd_defconfig so that
this target could be used to boot both mainline and the vendor kernel.

This makes maintenance task easier and avoid potentially confusion
for the end user.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>