.imxcfg configuration files support few more commands, all starting
with "hab_encrypt" string. That way it is possible to easily ignore
these commands, when image encryption was not requested. Hence, we can
use single .imxcfg file to generate signed and encrypted images in the
same build.

Images are encrypted in place by Freescale Code Signing Tool (cst),
using Data Encryption Key (DEK). This key needs to be encapsulated
by processor's hardware encryption engine to produce DEK blob, which
is unique for each device. DEK blob needs to be part of CSF area,
so we make enough space on the end of image to simply append it later,
e.g. during device flash procedure.

Introduced code was developed and tested on NXP i.MX6UL platform.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>