x86: theadorable-x86: Generate and pass root=PARTUUID instead of /dev/sdaX

To enable the root device selection (kernel cmd-line) via PARTUUID, this
patch enables CMD_PART on all missing theadorable-x86 boards and
changes the default environment to generate the root=PARTUUID string
automatically.

This fixes problems that have been noticed on systems with multiple
SATA/AHCI controller connected via PCIe, where the device name for the
root device / partition (/dev/sdaX) was incorrect.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>