uboot-imx

sh: Add support zimageboot command for Renesas SH

Curent U-Boot can boot zImage by use the "go" command.
But this is not right method. And this method can not set command-line
to linux kernel.
zimageboot sets command-line in environment of u-boot in linux kernel,
and provides function to boot it.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>