cmd: part: Add partition-related type command

cmd: part: Add partition-related type command

This implements the following command:

    part type mmc 0:1
      -> print partition type UUID
    part type mmc 0:1 uuid
      -> set environment variable to partition type UUID

"part type" can be useful when writing a bootcmd which searches for a
specific partition type to enable automatic discovery of partitions and
their intended usage or mount point.

Signed-off-by: Enric Balletbo i Serra <eballetbo@redhat.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
[trini: Fix when CONFIG_PARTITION_TYPE_GUID is disabled and have the
command check for "types" before "type"]
Signed-off-by: Tom Rini <trini@konsulko.com>