Add 'partition' options to 'save' and 'verify' commands

[lurch]

Currently the picotool load and picotool erase commands have a -p, --partition option to target just a specific partition.

For symmetry, it seems like the picotool save and picotool verify commands also ought to have a way to target a specific partition! (Although picotool save already has an existing -p flag, which is unfortunate.)