doc: imx: habv4: Add Secure Boot guide for i.MX6 and i.MX7 SPL targets

The current U-Boot implementation includes SPL targets for
some NXP development boards:

- mx6sabreauto_defconfig
- mx6sabresd_defconfig
- mx6ul_14x14_evk_defconfig
- mx6ul_9x9_evk_defconfig

Add additional steps needed to completly secure the
bootloader image.

Signed-off-by: Breno Lima <breno.lima@nxp.com>
1 file changed