board: tbs2910: Add CMD_PART

There is no stable mmcblk device numbering over different linux versions.
Enable CMD_PART to be able to query the UUID of the root filesystem partition.
So we can pass root=PARTUUID=XXX instead of root=/dev/mmcblkXpY in bootargs.
Leave the default environment as is for now to stay compatible with original
TBS settings.

Signed-off-by: Soeren Moch <smoch@web.de>