Code review comment for lp://qastaging/~mterry/phablet-tools/unlock

OK, I've reverted this branch to the assume-readwrite-image version. I'd really prefer not to bundle all these features together. This change is blocking landing the split greeter, so I'd like to make this as targeted as possible (this set of merges is already taking on another of your wish list items to move the logic out of ubuntu-test-cases into phablet-tools).

I've added a TODO about readonly support. Ready for review.