Fix mxc_hab documenation for DEK blob generation

Include/fsl_sec.h defines sec_in and sec_out, according to the
platform's endianess. Therefore, CONFIG_SYS_FSL_LE needs to be
declared in the configuration file of the target, in order to use
enable the DEK blob generation command. This requirement is not
explicit in the README.mxc_hab.

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