Enable individual pkg to be upgraded without rebuilding rootfs

[deitch]

NB: This probably should go in lf-edge/eve, but let's leave it here in the "testing world" for now.

As discussed in #22, developers working on a cycle will want to have as rapid iteration as possible. Specifically:

1. Build a pkg locally and run it on their dev env, without needing an EVE device at all, to iterate super-rapidly. When this is done...
2. Deploy it to a live EVE device, and re-deploy and re-deploy. When this is done...
3. Build rootfs and deploy, and re-deploy and re-deploy. When this is done...
4. It is ready!

We are working on the stage 1 above. Stage 3 is how we do it today. We need a stage 2: have the ability to deploy a pkg without upgrading EVE.

To be clear, this must be for testing only, and we need ways to ensure that this cannot go past it. It is an inviolable EVE design that the baseOS is immutable. But the middle stage is important to making development cycles quick and light.