mxsimage can now build unencrypted images with -u, so make use of it in
the Makefile. To keep the existing rules simple, name the generated
images *.mxsbsu instead of *.mxsbs.

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