warp7: Use PARTUUID to specify the rootfs location

warp7 can run different kernel versions, such as NXP 4.1 or mainline.

Currently the rootfs location is passed via mmcblk number and the
problem with this approach is that the mmcblk number for the eMMC
changes depending on the kernel version.

In order to avoid such issue, use UUID method to specify the rootfs
location.

Succesfully tested booting a NXP 4.1 and also a mainline kernel.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>