Support for changing the label of a partition.

[robert-hh]

This is not an issue, just a suggestion.
mp-image-tool-esp32 is very useful.
It would be a little bit more convenient if there were an option to simply rename the label of a partition. A use case would be to change the name of the file system between ffat and vfs. At the moment this is already possible by deleting the partition and adding it again with a different label.

[glenn20]

Ah, sorry @robert-hh I thought I had implemented this, but had misled myself. Implementation being pushed now.