Normally MXS SoCs only allow to boot images which have been
encrypted with a zero key. If the ENABLE_UNENCTRYPTED_BOOT fuse
is blown then the SoC also allows to boot unencrypted images.
Add an option to the mxsimage tool to generate such images.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Roland Hieber <r.hieber@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>