imx6: bootscript: use partition UUID for rootfs if possible

Specifying root filesystem by device is non-deterministic for several
reasons:
 - USB device unmeration order is not garunteeed for USB storage devs
 - MMC devs ordering is determined by the instance of the MMC host
   controller including non-storage SDIO devices which can throw off
   numbering depending on kernel versions.

It is recommended to use partition UUID

Signed-off-by: Tim Harvey <tharvey@gateworks.com>