mxs: improve image generation support

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Signed-off-by: Zoltan HERPAI <wigyori@uid0.hu>



git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38406 3c298f89-4303-0410-b956-a3cf2f4a3e73