part: Allow setting the partition-table type

part: Allow setting the partition-table type

Some devices have multiple partition types available on the same media.
It is sometimes useful to see these to check that everything is working
correctly.

Provide a way to manually set the partition-table type, avoiding the
auto-detection process.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>