disk: expose partition type flags

disk: expose partition type flags

GPT partition tables include two bytes worth of vendor defined
attributes, per partition. ChromeOS and Qualcomm both use these (with
different encoding!) to handle A/B slot switching with a retry counter.

Expose these via the disk_partition struct so that they can be parsed by
the relevant board code.

This will be used on Qualcomm boards to determine which slot we're
booting on so that we can flash capsule updates to the correct one.

Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>