For some usecases it is desired to still update the GPT headers even if
the UFS BSG device is missing. Add a new flag which implements this
behaviour.

It isn't made the default because on some platforms this could result in
an unbootable device, there it's preferable to defer to the user by just
bailing out if the UFS BSG device is unavailable.

Signed-off-by: Caleb Connolly <caleb@connolly.tech>