rockchip-update-img: add 'rootdelay=1' parameter in CMDLINE

Reduce boot delay from 30 seconds to 1 second, caused by rk30xxnand-3.0.36+.ko
which is not responding to wait-for-root in pre_mountroot() function of initrd
https://github.com/radxa/initrd/blob/master/scripts/local#L16