Add partuuid device referencing support to mender-convert

There has been work undertaken on this in meta-mender, and there is partial support for this in mender-convert with the grub.cfg containing variables that can be set as partuuids, however it looks like higher level configuration support needs adding to mender-convert to allow a user to use partuuids easily to reference the device parts

https://hub.mender.io/t/mender-convert-activate-partuuid-option/2110/3

https://tracker.mender.io/browse/MEN-2050
https://tracker.mender.io/browse/MEN-2051
https://tracker.mender.io/browse/MEN-3701

https://github.com/mendersoftware/meta-mender/pull/641/files