Rootfs size overrides maxsize for core-image-sato

@Austriker and @mirzak,
The instructions for growing rootfs on tegra are different and not well documented.
I was in the process of cleaning up the rootfs size calculations for tegra to make growing the rootfs easier when I found the issue described in U-boot environment located past 0xFFFFFFFF fails which I think will block you as well if you need a rootfs size > 4G. I haven’t tried sato yet but will try after I get these changes working, and will push a working branch this morning in case you want to try the latest or help with changes.