Add PARTUUID= PARTLABEL= support to datapart kernel parameter

[EmilyShepherd]

kiOS supports the datapart kernel parameter to specify where the data partition is. Currently this can either point to a block device file, or have the value of "auto", meaning that kiOS should match based on partition type.

It would also make sense if kiOS were able to support datapart values specifying the partition by PARTUUID or PARTLABEL as is relatively standard.

NB: PARTNOFF should not be supported, as this is pointless to support on a GPT-only system.