uboot-imx

Add 'true' and 'false' commands

These commands are only enabled when the hush shell is enabled and can
be useful in scripts such as:

while true do
    echo "Booting OS...";
    run $bootcmd;
    echo "Booting OS failed";
    sleep 10;
done

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>