Fix mxc_hab documenation

It is necessary to modify the configuration file for the target
board. It wasn't well documented that to enable any of the secure
boot modes, it is required to add CONFIG_SECURE_BOOT to the board
configuration file.

Also, fixed a typo in the encrypted boot section.

Signed-off-by: Ulises Cardenas <Ulises.Cardenas@freescale.com>