mx7_common: add secure boot support

Selecting the proper options to enable the build of the HAB tools.

Note, this support is disabled by default, one will have to select
the SECURE_BOOT configuration through menuconfig to enable it.

See doc/README.mxc_hab for more details.

Also remove duplicate options from board config headers.

Cc: Stefan Agner <stefan.agner@toradex.com>
Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>