mxs: reinstate rootfs image creation and dtb copy

Signed-off-by: Harald Geyer <harald@ccbib.org>
Acked-by: Michael Heimpold <mhei@heimpold.de>



git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45369 3c298f89-4303-0410-b956-a3cf2f4a3e73