warp7: select uuid partition based on rootpart

warp7: select uuid partition based on rootpart

Assigning the UUID discovery path to a tweakable environment variable means
that later steps in the boot process - particularly a boot script can
change the target root partition of a particular Linux boot.

Retargeting the rootfs is an important feature when doing ping/pong
upgrades allowing a boot script to select ping or pong as necessary without
reprogramming the bootloader.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Tested-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>