cmd_part: add partition-related command

This implements the following:

part uuid mmc 0:1
  -> print partition UUID
part uuid mmc 0:1 uuid
  -> set environment variable to partition UUID
part list mmc 0
  -> list the partitions on the specified device

"part uuid" can be useful when writing a bootcmd which searches all
known devices for something bootable, and then wants the kernel to
use the same partition as the root device, e.g.:

part uuid ${devtype} ${devnum}:${rootpart} uuid
setenv bootargs root=PARTUUID=${uuid} ...

Signed-off-by: Stephen Warren <swarren@nvidia.com>