ti_armv7_common: env: Use partuuid for detecting mmc root fs

Linux kernel can enumerate mmc sd as either mmcblk0 or mmcblk1.
But u-boot default environment assumes that sd always populates
as mmcblk0. With this the root fs is not being mounted when
mmc sd is enumerated as mmcblk1.
So use partuuid to update root= option in default environment.

Reported-by: Yan Liu <yan-liu@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>