mender-convert: rootfs overlay directory

In the "golden image" workflow, the user has to go through quite a few steps to add software to a running device, dump the device storage and generate an Artifact from it.

A more "professional" approach is to take a "base" (golden) image that is from upstream (e.g. Raspbian) and add a directory on top of it that contains the user application and any changes to the Linux system (such as which services to start on boot). This also includes the Mender client itself and configuration of it.

User value (why)

• Much easier and faster to generate new Mender Artifacts and Mender disk image files once the initial setup works
• Possible to update base image (e.g. kernel) independent of the application, as they are in separate "layers"
• Reproducible and understood images using supported upstream image and application layer on top

Size (SP)

• TBD

Acceptance criteria

• mender-convert takes a directory as input, which it copies on top of the root file system before generating Mender Artifacts and disk image
• any existing files with the same name get overwritten by this layer in the output image
• the permissions in the files of the directory are kept the same