part: Add accessors for struct disk_partition uuid

part: Add accessors for struct disk_partition uuid

This field is only present when a CONFIG is set. To avoid annoying #ifdefs
in the source code, add accessors. Update all code to use it.

Note that the accessor is optional. It can be omitted if it is known that
the option is enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>